Chosen Soldier is an account of how the US Army recruits and trains special forces soldiers. This was the state of affairs as of around 2004 and the basic structure is still the same. There is also a bit of history of special forces and unconventional warfare in general.
The 18X program, which was created in the last 10-15 years, is basically the equivalent of the SEAL challenge contract. Much of this book follows a class of "X-Rays", as they are typically called, through OSUT, SOPC, SFAS, and the Q course. The real training is done in the Q course. SFAS is the 35 day selection course that determines who goes on to the main training course and who gets dropped or recycled. Selection is what I have heard is the "toughest" part of the entire program in that once you make it through that point your odds of success dramatically increase. Clearly the whole program has varying amounts of difficulty, but statistically selection is the weeding out point.
The book was well written and achieved its main goal of explaining how SF training and selection goes down. Obviously, there are many parts that had to be glossed over for security reasons, not to mention parts that are continually evolving which implies that what you read today is not necessarily what one would go through tomorrow.