There are so many in the game and I normally just use the same two or three. The DC sells loads that I never even bothered trying. Is there any guide on all the Grimoires and what kind of wizard each is best suited to?

As far as I know, there is no such guide. The thing is, there are a few grimoires that are widely considered the best (Grimoire of Vaporous Wizardry, Ninagauth's Teachings, Llengrath's Martial Masteries, Snakeskin Grimoire, Grimoire of Spark and Flame) and there really isn't much to write about the others. I personally think that the starting grimoire, i.e. Weathered Grimoire, is quite useful, and that's because of Arcane Assault.

You can find the complete list of grimoires in the wiki:
https://pillarsofeternity.fandom.com/wiki/Pillars_of_Eternity_II:_Deadfire_grimoires
also with specific abilities (spell) onlyh contained in grimoires that you can't learn.
I don't remember any guide, you should equip the best grimoire(s) that fit best with your character and/or that are best for enemies you are going to fight.

https://pillarsofeternity.fandom.com/wiki/Llengrath's_Martial_Masteries

This is the best one for the melee Wizards imo.

Just use the wiki to find out the combination that needs least grimoire switch for you.

The one with unique spells are mostly niche. Iron-Clapsed is the most useful, giving Crushing Doom and Draining Missiles. Ninagauth's Teachings' unique spells aren't as good, but it has a good selection, and is completely free without any combat. Zandethus' Draconic Fury is great on a spellblade, and maybe soulblade/wiz?

This is an old topic that give you all the unlearnable spells, 

beside that most of grimoires have a subclass flavour, it is a good ay to categorize them.