Cripps book is the best (in my opinion).
You may also try:
High Linearity Power Amplifier Design, by Kennington.
It is abig bbok, it describes power amplifiers from A to Z, with focus later on linearization techniques (which is the challenging part in RF PAs).
But for real fundamentals, and deep treatment (though mathematically and conceptually very intense), I recommend Stve Cripps book.
There are other books out there, but many are highly specialized in sub-branches in RF PAs, like: Feedforward Power Amplfier Design for Multicarrier Applications..etc.
Go to Artech-house website (the main publisher of most good RF books), and search for RF Power Amplifier, you will get a handfull of titles..