When J.K. Rowling first put pen to paper (or finger to key - we're not sure how she got started) and wrote the very first 'Harry Potter' book, she would have hoped to come up with something exciting and entertaining, and profitable enough to tide her over until she could complete her next book. There's no way she could have imagined she was taking the first step towards a multi-billion-dollar empire. That's what happened though, and as with anything with a huge and obsessive fanbase, rarities are worth money.
The very first book, 'Harry Potter and the Philosopher's Stone,' was released in the US as 'Harry Potter and the Sorcerer's Stone.' If you can find a first edition of the book - with the non-American title and the author credited as 'Joanne Rowling' instead of 'J'K,' then you're looking at a retail value of $55,000.