Like many of the people posting on this thread, I keep my books in categories (baby, applique, techniques, precuts, Christmas, serger, scrappy, etc.) then by author's last name in each category. But what I do differently is to type each category into my label-making machine and print out the category name. Then I affix the label to the bookshelf so I can locate that group easily. I also have a master list in my computer which I update faithfully every time I purchase something. I take this list with me when I shop to prevent duplicate purchases.