Chords. Some people like them.
If you're one of those, then this is the only book you'll ever need. It'll tell you how to put chords together and then string one after the other until you have a "song." It's got bits about copping classic progressions from old songs and bits about making up your own. It's small, about 40 pages, and will fit in your gig-bag. The hand-drawn cover art is cool AF, because it's got all these overlapping rectangles in it, which is probably significant to the stuff inside, about music notes overlapping and combining and stuff.
You should buy this book, if you can find it somewhere. Start spouting out obscure jargon like "dominant chord" and "submissive chord" and get a leg-up on all those know-nothing jerk-wad musicians in your band. Show 'em who's in charge!