I dont think there is one "best" book on James "Whitey" Bulger but there area few excellent books.

The first book I read on all of this was BLACK MASS (Lehr and O'Neil). when I first read teh book it was a cant put it down book. I have read it 2 more times since then so here are 2 key observations
1. Keep in mind that the reporters wrote the book with a very biased view based on information fed to them by the disgraced ex-FBI agent John Morris. BLACK MASS has some glaring errors in the book once you do the research.

2. I felt at times that the book got very repetitive and could have been better with about 50 pages less IMHO. With that said I still highly recommend the book.

The book HITMAN (Carr) has stories about Whitey bulger but it centers on John Martarano. I say this book is EXCELLENT and MUST READ. This book has so many detailed stories its just riveting.

As mentioned in previous posts WHITEY (Lehr and O'Neil) is an excellent breakdown of who James Bulger was and how he became who he ultimately became, a gangster.

Finally, for quick reads and entertainment I would pick up RATMAN (Carr) and RIFLEMAN (Carr). RATMAN focuses on Whitey Bulger and RIFLEMAN on Steve Flemmi (Bulgers main partner in crime).

ALL of these books are highly recommended and grab em at the library before purchasing. Good luck