What is the best book to learn Oscillators Design ?

It might be this book for basic concept

Oscillator Design and Computer Simulation
AUTHOR: Randall W. Rhea
FORMAT: Hardcover, 320 pages
PUBLISHED: 1995
ISBN: 1-884932-30-4
Our Price: S69.00

Oscillator design is a specific subject that is referenced to your application. So question will be more specific and detailed. For instance 100MHz oscillator design principals can not be met with 5GHz oscillator design..
So , a detailed investigations should be performed subject to your application.

As big boss said depents your application and your technique. Although Oscillator principles are the same in MCW RFIC and RF the techcniques used are not.

Good books i can suggest are:

Modern Communication Circuits 2nd Ed (maybe there is a newer)
Jack R. Smith
Mc Graw Hill
ISBN: 0-07-115586-4

Microwave Transistor Amplifiers Analysis and Design 2nd Ed (thats what i have there are newer editions as well)
Guillermo Gonzalez
Prentice and Hall
ISBN: 0-13-254335

Fundamentals of RF Circuit design with Low Noise Oscillators
Jeremy Everard
Willey
ISBN: 0-471-49793-2

The design of CMOS radio-frequency integrated circuits,
Thomas H. Lee.
Cambridge ; New York, NY, USA : Cambridge University Press, 1998.

RF microelectronics,
Behzad Razavi.
Upper Saddle River, NJ : Prentice Hall, c1998.

The above books cover a big frequency range and applications:

now , I want to learn about basics , I think the first book is a good one now