I found this slightly-battered trade paperback at the Used Book Superstore, and as I'd enjoyed the Raffles stories I was glad to get this for a release copy.
Hornung, who happened to be Sir Arthur Conan Doyle's brother-in-law, wrote the Raffles stories as a sort of homage/dark-side version of Sherlock Holmes; Doyle had mixed feelings about this, admiring the writing but worried about making a criminal into the protagonist.
Here we first meet Bunny Manders and learn how he encounters Raffles, who'd been a much-admired senior boy when they were at school. Bunny is in dire straits, having bet money he doesn't have, and as Raffles is one of the people he lost to he's gone to ask for - well, he isn't sure; just to explain and confess before he blows his own brains out. But to his surprise Raffles laughs it off, admitting that he's short of cash himself, and suggesting a way to correct the situation. To Bunny's shock, Raffles sketches a plan for a robbery, but such is his desperation - and his unwillingness to be seen as less than "game" by Raffles - he goes along. And thus begins their partnership, and the saga of Raffles the "gentleman thief". [Raffles' own rationale: "Of course, it's very wrong, but we can't all be moralists, and the distribution of wealth is very wrong to begin with."]
Bunny's conscience never does quite give up trying to turn him away from Raffles' periodic crime sprees, but it always seems to fight a losing battle - either the need for more money tips the balance, or Bunny's desire to prove himself to Raffles (to whom he rapidly becomes devoted, in a rather tormented way, admiring him yet greatly distressed by Raffles' apparent lack of scruples). It becomes clear to the reader that Raffles is aware of this and plays on it whenever he wants Bunny's help, and I admit I was often stunned at the extent of his duplicity - though to be fair he always puts himself at as much risk as anyone else, even though his own quickness and luck seem to get him out of it while others may not be so lucky.
There's definitely a stronger push-pull of motivation and human desires in these stories than in the Holmes books, which are deliberately aimed at the clinical, emotionless side of things - that's probably part of Hornung's aim in creating the flip side of Holmes and Watson. While the Raffles stories are often funny, they're also emotionally draining and quite dark - and I've thoroughly enjoyed them.
[I found a collection of all of the Raffles stories, including those in this book and quite a few more: The Complete Short Stories of Raffles the Amateur Cracksman. If you enjoy the first set, it's worth looking up the rest.]
There have been several screen adaptations, including a 1975 mini-series, a 1930 film starring Ronald Colman, and a 1939 film starring David Niven. I've seen these and, while there are some good performances, none of them really present Raffles in his honorable-but-not-nice glory. Perhaps someone will make a definitive version someday...
[See the TV Tropes page on the stories.]