The Grand Grimoire is exactly the right name.
A Keeper may not literally need this book to run CoC magic well, but it makes it so much more likely, easy and fun. For Investigators its more of a list of even more things to worry about...
A compilation of 550+ spells from a 40 year back catalogue of CoC games; this really is as close to a real grimoire as you are likely to get. The spells are presented with variants, alternatives and twists already baked in - making it easy to individualise each one, just the way Cthulhu magic should be - unique, mysterious, individual, unknowable. This is not your grandfather´s spell list.
And horrific? Some of the spell features gave me actual shudders reading them, which is exactly what you want in this game. Not "oh this is an evil spell that needs a sacrafice" but "oh... that ... is a gruesome and nasty concept". Don´t give to anyone with trouble distingushing gaming fantasy from reality.
Word of warning - this is an encyclopedic volume of information, you will not absorb it all in one sitting. Far better to dip in from time to time and browse or look for something specific.
One thing that makes that a lot easier is the use of Spell categories - these list out all the spells with effects in a given field like "Making Monsters" so pick roughly what you want you villan to be interested in for example and then read through how they could accomplish it ... I defy anybody to do so without having a some ideas how they could use something from what follows in their campaign. The "Extending Life" list for example has given me long lived villans with vulnerabilities, perils of disloyal henchmen, complex preparations to interfer with, locked-room murder mysteries, vampiric red herrings, mulityear memory loss victims... in short plenty to investigate.
If pressed to find a flaw - its only that there isn´t a category for Gathering, Storing and Manipulating Magical Power ... those are included in Other Spells category. Might just not be enough spells written for that yet. And a Keeper doesn´t really need a "mechanically canon" explanation for high cost plot critical magic being cast anyway.
One thing on the wish list ... as the glyph/rune font pastes out to text ... could it be made available? Or even as a text "translation" website.
Anything else? Three very different but plausible NPC magicians to tailor into your game; a smattering of magical minions with statistics like Spectral Hunters and Hybrid Tindalosians; the normal high quality art work expected from 7e; and mystic diagram line art that would be frankly wasteful not to copy out for use in your own handouts.
In short, a wothwhile addition to any CoC bookshelf.