Your favorite math reference book(s)?

[Felix Gross]

Checked http://www.eurobuch.com (meta search engine for antiquarian booksellers):

The VNR Concise Encyclopedia is available. It covers many engineering math and pure math topics (number theory, measure theory, group theory etc.) at least briefly. Get the second edition.

If you are interested in old math books: archive.org offers many famous math handbooks as (mostly) very good pdf scans. Just for curiosity check out Briggs Log Tables from 1706.

Regarding the arcsinh terminology I checked out several reference. arcsinh is used quite often, as is sinh^-1 (that is real bad, since it is not 1/sinh(x)) and arsinh. No real consensus. The HP Prime manual uses ASINH which seems almost Solomonic.