What makes this book so amazing is that the actor who played Garak, Andrew Robinson, developed a detailed backstory for his character over the years as he was playing him. So what you're seeing in the book is based on what Robinson came up with to inform his performance as Garak during the run of the show.
IIRC, didn't he used to do a series of stage performances of Garak autobiographical monologues at conventions (prior to turning this into a book)? Seems like I had been hearing about this for years before the book, which made finally reading it an extra treat.
As far as I know he's also the only Trek actor to write a solo novel, without a professional writer co-author. (I could be wrong about that.)
Also its a really well-written novel.