The manuals themselves are great--most are hardcover and come loaded with incredible illustrations--but even the less flashy class handbooks can bring you valuable knowledge and inspiration when creating any characters that are or can relate to the abilities of clerics, druids, wizards, paladins, rangers, etc. etc. etc. (no but really, those are only a few.)
There are even multiple editions of the Tome of Magic, dedicated to detail, that will remind you of all things to take into consideration if you are creating your own magical school or society, or want a fleshed out list of magical items or spells to browse to get your imagination running in the right area of the cosmos for your own story.
I never considered crossing the D&D/horror-writing streams until one fateful afternoon in which I read a single paragraph about a particular monster, then ended up having an entire novel outlined by the time night fell. I'm still currently in the process of drafting it, but I think it's definitely safe to say that it is the most inspired of any idea I've had, and it's all thanks to that radical Monster Manual.
*dreamy sigh* Oh, Dungeons and Dragons. You are like an energy drink for the imagination.