I organize by theme and then by author within each group. On my computer I have 2 lists, one with all the books in alphabetical order by author, then name, and the theme in parenthesis, and the second one by themes with the books in alphabetical order under the theme heading. It was a pain setting it up but now when I get a new book I try to immediately enter it in both lists. It takes an extra minute but has helped me on several occasions.
I do the same for patterns except instead of author I use the publishing company's name.