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I have a chart of the Sutta Nipata I am using that lists the Khuddaka Nikaya as having 10 books.
The Wikipedia page for the Khuddaka Nikaya states there are 15 books ( Sri Lanka, Thailand) and 18 books (Burma).
Are the last 5 - 8 books considered to be apocryphal?
Several years ago I was looking for a digital translation of the PATISAMBHIDAMAGGA (THE PATH OF DISCRIMINATION) and I could only find a scan of a very old Pali Text Society translation.
Why aren’t there are more modern translations?
My impression is the following books are belated:
Possibly because it is uninteresting. When I browsed it, it appeared mostly lists of the dhammas from the suttas. The explanation of Step 3 of Anapanasati seems useful. What I did find interesting was prominent terms in the Visuddhimagga seem to first appear in the Patisambhidamagga, namely, ‘patisandhi-vinnana’, ‘kama-bhava’ & ‘upapatti-bhava’. It is also notable the Abhidhamma Vibhanga refers to ‘kama-bhava’ & ‘upapatti-bhava’ as being Sutta teachings. It is also interesting the same Abhidhamma Vibhanga seems to not mention ‘patisandhi-vinnana’. The above said, while the relevant paragraphs are vague because they exclude jati (birth) & marana (death), my impression from the Patisambhidamagga is ‘patisandhi-vinnana’ does not refer to a ‘rebirth consciousness’ but simply a present life/moment continuation of consciousness.
Possibly a Pali literate here can translate the following passages for us: 
Purimakammabhavasmiṁ moho avijjā, āyūhanā saṅkhārā, nikanti taṇhā, upagamanaṁ upādānaṁ, cetanā bhavo. Ime pañca dhammā purimakammabhavasmiṁ idha paṭisandhiyā paccayā. Idha paṭisandhi viññāṇaṁ, okkanti nāmarūpaṁ, pasādo āyatanaṁ, phuṭṭho phasso, vedayitaṁ vedanā. Ime pañca dhammā idhupapattibhavasmiṁ purekatassa kammassa paccayā. Idha paripakkattā āyatanānaṁ moho avijjā, āyūhanā saṅkhārā, nikanti taṇhā upagamanaṁ upādānaṁ, cetanā bhavo. Ime pañca dhammā idha kammabhavasmiṁ āyatiṁ paṭisandhiyā paccayā. Āyatiṁ paṭisandhi viññāṇaṁ, okkanti nāmarūpaṁ, pasādo āyatanaṁ, phuṭṭho phasso, vedayitaṁ vedanā. Ime pañca dhammā āyatiṁ upapattibhavasmiṁ idha katassa kammassa paccayā. Itime catusaṅkhepe tayo addhe tisandhiṁ vīsatiyā ākārehi paṭiccasamuppādaṁ jānāti passati aññāti paṭivijjhati. Taṁ ñātaṭṭhena ñāṇaṁ, pajānanaṭṭhena paññā. Tena vuccati— “paccayapariggahe paññā dhammaṭṭhitiñāṇaṁ”.
I was searching the term “patisandhi” and found it in the Therāpadāna 8. This text is interesting because it says:
Paṭisandhiṁ na passāmi,
vicinanto bhave ahaṁ;
Nirūpadhi vippamutto,
upasanto carāmahaṁ.I witness no re-becoming (patisandhi);
I’ve investigated being;
free of desires and fully free,
calmed, I’m wandering about now.Suttantaṁ abhidhammañca,
vinayañcāpi kevalaṁ;
Navaṅgaṁ buddhavacanaṁ,
esā dhammasabhā tava.Your court of law consists of the
nine-fold teaching of the Buddha,
the Suttas and Abhidhamma
and the whole of the Vinaya .
While I imagine any text could have been composed anytime, the above gives the impression the Abhidhamma was prominent when the Apadana was composed.
The term ‘patisandhi’ is found in the Patisambhidamagga below:
Purimakammabhavasmiṁ moho avijjā, āyūhanā saṅkhārā, nikanti taṇhā, upagamanaṁ upādānaṁ, cetanā bhavo. Ime pañca dhammā purimakammabhavasmiṁ idha paṭisandhiyā paccayā.
In past action becoming there is delusion, which is ignorance; there is accumulation, which is formations; there is attachment, which is craving; there is adoption, which is clinging; there is volition, which is becoming. These five things here in past action becoming are the relinking condition.
Idha paṭisandhi viññāṇaṁ, okkanti nāmarūpaṁ, pasādo āyatanaṁ, phuṭṭho phasso, vedayitaṁ vedanā. Ime pañca dhammā idhupapattibhavasmiṁ purekatassa kammassa paccayā.
Here, there is relinking, which is consciousness; there is emergence, which is mentality-materiality; there is sensitivity, which is sense bases; there is what is touched, which is sense contact; there is what is felt, which is feeling. These five things here in upapatti (resultant; rebirth) becoming have their conditions in action done in the past.
An equivalent to paṭisandhi viññāṇaṁ in the Abhidhamma Vibhanga may possibly be vipākaṁ viññāṇaṁ. The Vibhanga says:
Katame dhammā abyākatā? Yasmiṁ samaye kāmāvacarassa kusalassa kammassa katattā upacitattā vipākaṁ cakkhuviññāṇaṁ uppannaṁ hoti upekkhāsahagataṁ rūpārammaṇaṁ, tasmiṁ samaye saṅkhārapaccayā viññāṇaṁ, viññāṇapaccayā nāmaṁ, nāmapaccayā chaṭṭhāyatanaṁ, chaṭṭhāyatanapaccayā phasso, phassapaccayā vedanā, vedanāpaccayā bhavo, bhavapaccayā jāti, jātipaccayā jarāmaraṇaṁ. Evametassa kevalassa dukkhakkhandhassa samudayo hoti.
What states are neither-good-nor-bad? At the time when having done, having accumulated bad action there arises resultant eye consciousness accompanied by indifference, having visible object; at that time because of bad roots there is activity; because of activity there is consciousness; because of consciousness there is mind; because of mind there is the sixth base; because of the sixth base there is contact; because of contact there is feeling; because of feeling there is becoming; because of becoming there is birth; because of birth there is ageing and death. Thus is the arising of this whole mass of suffering.
In summary:
The concept of ‘paṭisandhi’ can be found in the Questions of King Milinda.
Here, the two ideas of saṃkamati & paṭisandahati are discussed with a simile of lighting a lamp:
https://suttacentral.net/mil3.5.5/en/tw_rhysdavids?reference=pts&highlight=true
Rhys-Davids renders saṃkamati as ‘transmigration’ and paṭisandahati as ‘rebirth’.
I’m not sure if these concepts are used in the main Nikayas. I can’t think of any.
I believe Ven. Bodhi renders paṭisandhi as ‘rebirth linking’ (consciousness) in an Abhidhamma context.
Yes, which seems again useful for gaining a chronological impression of the texts.
As time went by certain concepts were fleshed-out, elaborated, etc.
In your Patisambhidamagga quote above, the word ‘pasādo’ (‘sensitivity’ or sense-base) seems to appear like this in Abhidhamma, but not in the Nikayas.
New Concise Pali English Dictionary
pasāda
masculine
PTS Pali English Dictionary
pasāda
The Abhidhamma Vibhanga has the usage below, translated as “faith”:
Having done, having developed that same good supramundane jhāna, he, aloof from sense pleasures, attains and dwells in resultant first jhāna that is hard practice, knowledge slowly acquired and is empty; at that time because of good roots there is activity; because of activity there is consciousness; because of consciousness there is mind; because of mind there is the sixth base; because of the sixth base there is contact; because of contact there is feeling; because of feeling there is faith; because of faith there is decision; because of decision there is becoming; because of becoming there is birth; because of birth there is ageing and death. Thus is the arising of these states.
Tasseva lokuttarassa kusalassa jhānassa katattā bhāvitattā vipākaṁ vivicceva kāmehi …pe… paṭhamaṁ jhānaṁ upasampajja viharati dukkhapaṭipadaṁ dandhābhiññaṁ suññataṁ, tasmiṁ samaye kusalamūlapaccayā saṅkhāro, saṅkhārapaccayā viññāṇaṁ, viññāṇapaccayā nāmaṁ, nāmapaccayā chaṭṭhāyatanaṁ, chaṭṭhāyatanapaccayā phasso, phassapaccayā vedanā, vedanāpaccayā pasādo, pasādapaccayā adhimokkho, adhimokkhapaccayā bhavo, bhavapaccayā jāti, jātipaccayā jarāmaraṇaṁ. Evametesaṁ dhammānaṁ samudayo hoti.
Example
Yes, my point is at the end of entry #1.
The word ‘pasāda’ is related to the verb ‘pasīdati’, and its past participle ‘passana’.
(Skt prasāda)
So there is the connection between ‘brightening up’ , ‘gladdening the mind’, and having faith.
In the Abhidhamma, ‘pasāda’ takes on the meaning of ‘sense-base’.
As a side note, my personal impression of the PTS dictionary it is saliently interpretive rather than etymological. 
Interesting details about the usage of patisandhi. It does suggest that these late books of KN might even be later than the Abhidhamma, although of course we’d need many more data points to reach a conclusion.
Meanwhile, to answer the original question. These later books generally don’t attract the same level of interest as the EBTs, but still, they should be present as there is always something valuable to learn.
Most of them do have good translations via the PTS, although the later translations are still sometimes encumbered by copyright. Good news, though, the Patisambhidamagga has been made available by them and is in preparation for SC. This is the Nyanamoli translation, which like all his work, is highly technical and precise, which is very suitable for such a work.
I am looking for an English translation of Paṭisambhidāmagga. Would you be open to sharing the scan of Paṭisambhidāmagga that you have?
No need, it’s on SuttaCentral now thanks to Manfred and Ayya Vimala, and with support by the PTS.