I saw this the other day and only just now had time to actually respond:
Name your three biggest non-reference books (excluding the Bible and text books):
The Libretti of the French, Italian, and German Operas with Commentary and Musical Annotations
A La Recherch du Temps Perdu a single volume French Edition
The Riverside Shakespeare (Was a text-book, but isn't anymore--worn to a frazzle with years of reading).
Name your three biggest reference books:
Treatise on Invertebrate Paleontology--Volume S Echinodermata
The Oxford Classical Dictionary
A Compendium of Syro-Phoenician, Akkadian, Sumerian, Egyptian, and Other Ancient Sources