Suttas. 
Outside of suttas I think it is impossible to have best resource for everything, if you want to delve into interpretations you need to compare various stuff on interesting you subjects and seek what resonates with you. There will always be people preffering approaches of different various teachers.
Because of that, I think Bhikkhu Sujato & Bhikkhu Brahmali - Authenticity of Early Buddhist Texts is great book, because it points you straight to the suttas and tell you which are authenthic.
authenticity.pdf (1.4 MB)
Also, very importaint book for me was…
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Bhikkhu Sujato - Swift Pair of Messengers
Bhikkhu Sujato - A Swift Pair of Messengers.pdf (1.3 MB)
…Because it dispells all most importaint conflicts around various interpretations and shows imo most correct interpretation of the suttas. It is also full of amazing quotes from the suttas. You won’t get lost on the path with this book as a guide. Thank you Ajahn @sujato for writing it, Sadhu Sadhu Sadhu! 
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Bhikkhu Nanananda - Deliverance of Heart
Deliverance_of_Heart.pdf (1.0 MB)
My favorite book on metta. Shows practice of metta in really spiritual, altruistic, humble, non self-centered way. Also full of sutta quotes. It is also a great remedy for spiritual conceit.
With Metta 