I’ve browsed through several and bought one bike touring book. I have never gotten any worthwhile information from any of them.
I think the main problem is that there’s so much difference between all the different types of bike touring you can do. You can tour with a $50 heap of crap or $5000 custom handmade bike. You can tour for 2 nights or 2 years. You can tour while stealth-camping and cooking with a camping stove every night, or staying in hotels and eating at restaurants every night. You can tour fully loaded or with a sag wagon following you. You can tour alone or in a tour group. You can ride 30km a day or 300km a day.
I think the best way to teach yourself about bike touring is to 1) go to a local bike shop to figure out what kind of rudimentary gear you need – a cheap mountain bike with a rear rack and a pair of panniers are more than enough to get you started, and 2) go bike touring a few times, starting off with overnight tours.
Keep the rubber side down.