Related Clues:

• Multi-vol. library references
• Weighty refs
• Refs. that may occupy whole shelves
• Brit. reference sets
• Library references, briefly
• Voluminous syn. and etym. sources
• Ref. works that can run $1,000+
• Some humongous ref. books
• Etymologist's vols
• Big ref. volumes
• Massive ref. books