Is there any book you have bought that made you wish you could travel back in time to prevent yourself from purchasing it?
Because it just didn't live up to your expectations? Or because it seriously upset you somehow?
Mine would be, and that might surprise some, not be Unseen Academicals, but Folklore of Discworld, which I bought after reading the 'note from the publisher' in UA. As a more casual Dicworld fan I went and bought the book in hope it would explain some things UA didn't deliver an explanation for. But, oh surprise, the needed information were NOT in the version of Folklore that was out at the time, no, those came in a new edition much later on. (which I didn't buy after using amazon's 'search inside' feature...only to discover that a trip to wikipedia and looking up 'orcs' 'goblins' and 'medieval football' provided the same, if not more and better information)
The second book I actually regret buying even more is Jo Walton's Tooth and Claw.
The text on the back promised a book that combined one of my all time favourite themes, th victorian age, with one of my crazes at the time: dragons. It promised a world full of dragons living in a victorian setting.
could have went well if Miss Walton wouldn't have written a cheap penny romance novel with a semi-historical setting and just replaced the word 'human' with dragon and sparsely sprinkled it with some dragon-specific things here and there. What I got was a book in which dragons of 20m and more in size ride trains in which they sit on banks, ride coaches drawn by 4-8 regular horses etc. There is only that much suspension of disbelief I can muster.
Combine that with a very boring and clichéd standard plot and you have a book you can only regret buying.