So, I'm just going to put it out there: I have zero self control when it comes to impulse buying stuff. It's terrible. So, often I'll grab a gaming book because at that moment I want it, even if it has no long term application. I justify it by saying "I'll use it some day in some campaign down the line." I've bought scores of books that almost immediately became self decoration. Not that they weren't good games, necessarily - they just got a read through and then never saw use. Pendragon really stands out when it comes to being a great game that I bought, read and never played. Then there are some of the more obscure (or at least less played) games - Dogs in the Vineyard, Secrets of Zir'an, Monte Cook's Arcana Evolved, Lowlife, and Tales from the Floating Vagabond all spring to mind.
Then there are books I grab in a moment of impulse that see a ton of use and become staples of my collection. This came to mind because yesterday my copy of Monstrosities for use with Swords & Wizardry arrived. I was a bit remiss to pick up this book because of the hefty price tag. Besides, did I really need another book of monsters for my game? There were enough in my other OSR books to surely fill the gap, right?
Then answer is yes, that is absolutely true. But Monstrosities is still an awesome product. This 500 page tome is more than just another book o' monsters. Granted there's definitely some retread ground here - but there's also a lot of original stuff too. What really sells it is that each monster listing comes with an adventure or encounter hook which makes the book easily applicable at the table. Yeah, it was pricey, but so, so good.
Which brings me to the meandering point I was getting at: What are books you grabbed on a lark or a whim that turned out to be great gaming purchases?