There are some very loud voices out there on the webs, which is perhaps what Venerable Sujato is referring to. But I think it is fundamentally a good question.
The disagreement is on what defines a state of jhana. It focusses mostly on two things: whether there is any experience of the five bodily senses and whether there is analytical, verbalized thought.
The traditional Theravadin interpretation of the commentaries and I’d say also Abhidhamma is that both these things don’t exist in the jhanas. Some follow this interpretation, either depending on the commentaries or having arrived at that conclusion independently. Others say the commentaries had it wrong, and though they do not agree on what the jhanas are like, they generally agree there is feeling of the body and reasoned thinking in the jhanas. (Broadly these are the two interpretations, but of course some will fall in between, e.g. you can feel the body but not think, for example.)
For the latter, where there is body and reflective thought, you can see Richard Shankman’s - Experience of Samadhi. And recently Venerable Kumara’s book (still in draft, I belief) was discussed here a while ago.
For the former, I’m not aware anybody ever wrote such a book that specifically argues against the other interpretation. There’s bits and pieces here and there, like on Venerable Sujato’s old blog. (You’ll have to google yourself. 
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Much of the discussion hinges on the meaning of kāya, which can mean ‘body’ but also person as in English ‘somebody’. Both Pali Text Society dictionaries, for what it’s worth, suggest the jhana similes to be interpreted as the latter.
That’s the short of it.
I once replied to a question on another forum here: Deep Jhanas vs. Light Jhanas - Dhammaloka Community
And I’ve discussed it once on twice here on suttacentral, but for some reason this topic just seems to get under people’s skin more than others. Could be the way I phrase myself, of course. 
Either way, it wasn’t particularly productive so I’ll refrain from going there again.
There was also this video shared once on here.
Oh, by the way, I actually the question is usually asked the wrong way. Instead of asking what the jhanas are, we should ask ourselves what the hindrances are! Because underestimating the hindrances, that is the real problem, I think we can all agree.