I somehow avoided the whole Harry Potter phenomenon until the 4th book came out. Now I am reading the books to my son.
The question that has occurred to me is how fast did this Harry Potter craze take hold? Was the first book a smash hit right away or did it take a couple of books before Potter-mania started running wild? How long was it before Ms. Rowling knew she was sitting on a gold mine?
Actual information:
The first book was released in the UK in 1997, and was initially handled the way any first children’s book from an unknown author would be – not a huge number of books in the first run. A first edition, first printing of the first book recently sold at auction for 25,000 pounds. However, the book sold quite well for a children’s book, and quickly went into additional printings. Based on this success, a larger-than-usual printing was scheduled for the US release. The second book also sold extremely well for a children’s book, both in the UK and the US. At this point, Ms. Rowling would have been considered a success in her market.
It was with the third book that the market went absolutely insane. It became the fastest selling book ever in the UK, a record it held until the release of the 4th book.
Anecdotal information:
Just FYI, even when the first book came out in the UK, there was quite a bit of buzz about it in certain circles. (Those would be the high flying circles of children’s librarians and primary and middle school teachers and some folks in children’s publishing 
) In my children’s lit bookclub, the first book had so much talk associated with it that people in the US were ordering it through amazon.uk to get a jump on the US release date. I think the success of the series wasn’t so surprising to this crowd, it was getting great word of mouth from day one, much more so than any other children’s book in my memory, at least.