P. A. M. Dirac 
The Principles of Quantum Mechanics [EPUB ebook] 

‘The standard work in the fundamental principles of quantum mechanics, indispensable both to the advanced student and to the mature research worker, who will always find it a fresh source of knowledge and stimulation.’ –Nature ‘This is the classic text on quantum mechanics. No graduate student of quantum theory should leave it unread’–W.C Schieve, University of Texas