Do you have a book of sheet music you own that you value above the others? Maybe one you've learned tons of pieces out of? Or one you'd be really lost without?
My favorite book in my collection is my Dover "Complete Shorter Works for Piano" by Brahms. The binding is falling apart due to too much use and a couple pages in the Rhapsodies Op. 79 area are falling out as a result of over-enthusiastic page turns, but I love the book. Brahms was a genius and wrote many beautiful pieces. I've only learned a few, but I enjoy reading through the pieces in that book...someday, I'll learn every piece in it and be able to play them all at will, and play them so much people will say, "Honestly, David, don't you know anything other than Brahms?" The book itself is wonderful, too...really nice silky-smooth pages and dark, clear engraving. I really wish those pages were still attached to the binding though...

So, anyone else? Perhaps you have a collection of Beethoven sonatas you're fond of, or Chopin nocturnes? Or Rachmaninoff preludes?