Avoid opening books unless you have some specific purpose in mind.
My all-time favorite chess book is "Pawn Power in Chess" by Hans Kmoch, which must have added about 400 points to my playing strength... although that required a few re-readings.
Be aware that it is a very polarizing book... some folks (including some GMs) think it's one of the most profound chess books ever written. Other folks (including some GMs) think it's garbage.
WARNING! - This is not a chess book for limp-wristed, half-hearted dilettantes! It is a chess book for knuckle-dragging, hairy-backed MEN! The author will expect you to THINK!
I have started playing chess again after a number of years and keen to hear what people think are the key books (not specific opening books) that are on the must read list.