Historical clue formulations

• Massive ref. books — 2021-05-23
• Multivolume ref. works — 2020-08-20
• Ref. works that can run $1,000+ — 2019-01-06
• Library references, briefly — 2017-09-28
• Brit. reference sets — 2016-04-24
• Refs. that may occupy whole shelves — 2016-01-12
• Ref. books sometimes sold with magnifying glasses — 2015-06-15
• Meaningful sets, for short? — 2014-10-09
• Library refs. — 2014-04-23
• Ref. works sometimes sold with magnifying glasses — 2012-06-13
• Hefty refs. — 2010-04-22
• Lib. references — 2009-12-20
• They start A-B, C, D-E, etc. — 2009-02-08
• Multivolume refs. — 2007-04-18
• Ref. volumes — 2004-11-28
• Library refs. — 2003-04-30
• British refs. — 2002-08-05
• Multivolume ref. works — 1998-03-02
• English refs. — 1996-05-05
• Multivolume refs. — 1995-09-30
• Multivolume dicts. — 1994-11-08