This question comes up from time to time. I'll save you having to do a quick search though. Lennard Zinn's book "Zinn and the Art of Mountain Bike Maintenance" and "Bicycling Magazine's Complete Guide to Bicycle Repair and Maintnance" by Jim Langley are two highly regarded manuals.
And see
this thread for the index to a free, online, downloadable Barnett's Manual which has more technical information than a home mechanic will ever need.
Have fun!