I think the book to get depends on the guns you are interested in the most.
The motherlode of information about Smith & Wesson is Roy Jinks, factory historian. He has authored/co-authored 2 popular hardcover books.
The earlier one is "Smith & Wesson 1857-1945". It was written by Jinks with Robert J. Neal. An exhaustive, detailed tome about all S&W products made before 1945, and I mean detailed. It explains a lot of the minor and not-so-minor engineering changes including when they were written up as manufacturing instructions.
The second Jinks book is "HISTORY OF SMITH & WESSON" which covers both the earlier history of the company and it's products but also covers the guns made from 1945 to 1977. It includes the serial number ranges for different frame sizes and models.
These two books are the foundation of knowledge about our favorite firearms. Pretty well all later books by other authors either quote or plagarize the information in them; some authors acknowledge where they got their information, some don't.
Best book on S&W that includes later guns is "THE STANDARD CATALOG OF SMITH & WESSON" by Jim Supica and Richard Nahas. It is unique in that it includes estimates of what were current market values when it was published. It covers S&W products up to 2006. It is in it's third edition and we are all hoping the fourth edition will come out someday soon. These authors go into details other books haven't collected and published, such as boxes, grips and stocks and lots of practical advice for folks starting to collect or accumulate S&W's.
I can't imagine not having all three.