In your research you are directed towards a few tattered tomes on the subject, perhaps better described as journals.
They are written in the scrawled penmanship of a scholar obsessed - the contents a mixture of diagrams and careful, scientific observations and liberal guess work. The journals read almost as though the writer were at war with himself, trying desperately to understand something that defied all sense to uncover a truth that horrified and appalled him even as it intrigued him, drawing him deeper into his feverish obsession.

The most interesting excerpts tell the story of how the writer spent a week hiding in an "Illithid" settlement in the Underdark, masquerading as a human thrall and praying to all the gods that the enchanted ring he had purchased for himself would protect his mind as surely as the enchanter had promised.

There are a number of diagrams and careful drawings depicting the life cycle of the creatures, and some cursory notes on how they reproduce.
It appears they begin life as a small larvae, a tiny squid like creature with four tentacles, swimming in a pool with something called an "Elder Brain".

What follows is a gruesome depiction of a larvae being implanted in the ear of a sedated host. The larvae consumes the hosts brain and, through some parasitic process, restructures the host's internal organs and digestive tract to match mature illithid biology. The changes then manifest externally, with the head of the host becoming a larger version of the original larvae.

The mature creatures subsist on brain matter extracted from living victims with their tentacles. Their bodies maintain many of the racial and physical strengths of their original hosts.

Toward the end, the author's scratchings grow delirious with his thoughts having turned to mad paranoia. He was sure that everyone in his life had been replaced by Illithid thralls and that the mind-flayers spied on his every move. He even began to postulate that his wife had not merely been replaced by a mind controlled thrall, but that she herself had been replaced by an Illithid whose metamorphosis had been deliberately stunted to as to prevent the external evidence of the shift from manifesting. He claimed that the town watch had discovered a corpse with its skull cap and brain missing on several occasions and that his wife would disappear at odd hours with no explanation for her absence.

The writer's story ends abruptly with no closure to his tale. Perhaps his sanity was restored and he forsook his mad study. Perhaps he was quite sane and his research spelled his doom.
We may never know.