This article is from the Piano Purchase and Maintenance FAQ, by Isako Hoshino email@example.com with numerous contributions by others.
The general consensus is, "If you don't know what you are
doing, leave it to the professionals." Piano is a very
delicate instrument. You do not want to mess with it unless
you know what you are dealing with. If you carelessly fiddle
around too much, you may do more damage and end up with
costly repairs, and the last thing you want on your hands is
your piano irreparably damaged.
But for those who understand the risks involved and still
choose to pursue tuning and repairs themselves, there are
several books available on the subject.