The Bookcharmer's most recent post was, shall we say, snarky, so in karmic amends I post an example of what Choice considers Essential. The BC is smug to point out this title is already accessible as an e-book through the Library Catalog!
Here's (excerpt of course) what Choice has to say:
Encyclopedia of nineteenth-century photography, ed. by John Hannavy. Taylor & Francis, 2008. 2v bibl index afp ISBN 0-415-97235-3, $440.00. Reviewed in 2008mar CHOICE.
This monumental encyclopedia is a treasure trove of information covering all aspects of 19th-century photography. The scholarly essays, ranging from 200 words for minor topics to 5,000 words for main entries, are highly informative and often contain a bibliography for further reading. Readers may access information via a detailed index, an alphabetical arrangement of topics, or a more useful thematic listing. ... These two volumes will no doubt remain the standard reference work for 19th-century photography for many years. Summing Up: Essential. All collections; all levels. -- C. Larry, Northern Illinois University
Here's another nice example of what makes a book "essential": "No comparable work exists." this phrase is applied to:
Encyclopedia of privacy, ed. by William G. Staples. Greenwood, 2007. 2v index afp ISBN 0-313-33477-3, $199.95. Outstanding Title! Reviewed in 2007jun CHOICE.
This two-volume set presents information and analyses of privacy and privacy-related issues in 226 entries written by over 100 experts. It covers various historic and current aspects of the topic, including legal, political, social, and economic issues.... No comparable work exists. This excellent resource is more comprehensive and up-to-date than Privacy in the Information Age (CH, Jul'00, 37-6024). Summing Up: Essential. Lower-level undergraduates through faculty/researchers; general readers. -- M. M. Strange, University of Wisconsin--LaCrosse
Nice, aren't they? And there are 233 more Reference titles alone that earn the status "Essential" in the current database of Choice Reviews.