I hope the responses received to your question at the Google+ community help. Books get mentioned a lot in Lovecraft's tale - but, they're read far more rarely. Lovecraft seems to use them as a signpost to piles of worm-riddled, half-forgotten lore - but few spend much time actually consulting the texts. It isn't a failing in the Core Book that books don't get a mention - it's just that they're not core to the game, especially given that spells are really dangerous and likely, more often than not, found in the hands of cultists and their monstrous masters.
De Cultis Ineffabilibus - originally code-named Unaussprechliche Hack - the book of optional rules and ideas will feature mechanics for additional creatures, magic, pulpier combat and more, with marginalia and scattered notes. The magical part will touch on grimoire and tomes in more detail, but nothing major. The Cthulhu Hack has never been about emulating the Call of Cthulhu RPG - it's about resurfacing the works of Lovecraft based solely on interpretation of the text. The Cthulhu Hack looks to emulate the elements within the Mythos stories that Lovecraft wrote - or co-opted into his universe by editing them. Investigation and Sanity as a dwindling resource, for example, emulates how Lovecraft's protagonists become broken the more they expose themselves to the research and discovery of the unknown.
I hope that's helpful. More information to come - with some available now as PDFs-in-playtest to those who support The Cthulhu Hack Patreon.