Back when I was a wee lad, I loved the Basic and Expert D&D (not AD&D) manuals. It was great- page after page of imaginative monsters, spells, magical items, illustrated with images of people in the D&D world having adventures. To a kid in Smalltown, USA it was great stuff!
Now I find that Borges’ Book of Imaginary Beings is a bit like a D&D rulebook for grownups. Unfortunately it’s only about creatures, not spells or magic items, but there’s always Brewer’s Dictionary of Phrase and Fable.
What fires your imagination?