The "usual suspects" (Park Tool manual, Lennard Zinn's books) are excellent manuals (I have Zinn & the Art of Road Bike Maintenance), but for the beginner or "casual" mechanic I suggest a little book I picked up at Barnes & Noble one day: DK Bicycle Repair Manual by Chris Sidwells, Dorling Kindersley (DK) Publishing. It's glossy and may seem superficial but it's clear and covers the basics of all kinds of bikes - from adjusting derailleurs to maintaining the shocks on mountain bikes, to the coaster brake on your grandkid's bike.
It might be all you need (plus tools) for most routine maintenance and adjustments.