There's no one manual, although the help screen can be pretty helpful. Most
people use Project in many different ways, and it's tough to standardize that
into basic guidance.
That being said, there're a number of good books out there. I generally
recommend IIL's Dynamic Scheduling with Microsoft Project (2003 or 2007). MS
Project Experts' Ultimate Learning Guide to MS Project 2007 is also staring
at me from my bookshelf as I write this.
I'd also recommend a good training course from a competent instructor.
Typically ramping people up on MS Project is a 2-3 day training endeavor,
either in one shot, or split up over a couple of weeks - depending on your
background in project management and scheduling tools.