The Toronto Collection
While these books don't form a strict series, they are all set in the same area of Toronto and some of the earlier books' characters and locations do make appearances in later books. You wouldn't completely spoil your reading experience by picking them up out of order, but you would definitely find spoilers of how the earlier books end! To avoid that, start at the beginning of this list and read down. :)
Know you want the whole collection? Check out these electronic box sets of the first thirteen books in the Toronto Collection and save!