I think i've bought maybe 5 guides in my life. Often for anyone born after like 1987 the default issues could be solved with a visit to a site like GameFAQ's or an earlier equivalent.
But every so often i've picked one up because it is a well crafted encyclopedic archive of stats, drop rates and shit you would need to keep a tab open when farming. Stuff that isn't just a printed out low tier walkthrough. The Bloodborne guide was good for this. It has full maps and item locations but more important has stat weights for all weapons as you upgrade them and how they work with your stats, along with blood gem drop rates and from what enemies and little tidbits like the giants being called 'Lost Children' which once again ties an area into the whole choir/orphanage child experimentation stuff. Its neat but apparently miyazaki was writing extra stuff to make it seem like a hunters journal with stories and such but never got it to Future Press in time. Thats my average tier but some go beyond this for me. Two examples come to mind:
>Final Fantasy Chrystal Chronicles Official guide: Not only does this also have a total list of every item in the game -in a game where inventory slots and the like are end of dungeon drops thats pretty fuckin' important- but also has deep lore sections from the original writer explaining the exffects of Raems Miasma on the land and the history of places like the Necropolis that feel barely explained in the game. Its a great guide for the game with high quality art and design that matches the game throughout but is one of two guides i keep on my bookshelf because theres good reading in there for a diehard fan of the game.
also
>Grimoir Nier Complete: a JP only but fan translated guide for Cavia's swang song PS3 cult classic but also half a dozen light novels and interviews with Taro Yoko talking about the history of the world, its connections to the watchers, the red eye legion and other stuff that is barely touched on in the game and DoD fans felt were strangely absent. Its a great guide and the game is literally incomplete as an experience with its short stories.
What guides have you bought to hold onto for reasons beyond a walkthrough? whats your top tier guide?