Possible Clues:

• Funk & Wagnalls offering: abbr.
• Multivolume ref., often
• Multivolume ref. work
• Multivolume ref., usually
• Ref. set
• It comes in volumes: Abbr.
• Atlas shelfmate: Abbr.
• Dict. shelfmate
• Ref. work
• Britannica, for one: Abbr.