There are two different series of MKSAP. MKSAP was originally for internal medicine residents preparing to take the IM boards. The series consists of several books (one on each subspecialty of IM), containing both high yield info and a question bank. The series gets updated about once every 3 years. They are currently on MKSAP 14.
The MKSAP everyone here is talking about is the MKSAP for Students series. This consists of one book, filled with a whole bunch of questions designed to help medical students with their IM rotation, and a lot of people use it to study for the shelf. Currently, this series is in it's third edition, and the book to buy is MKSAP For Students 3.