I just looked at the main catalogue at www.nls.uk and it's not easy to find that book there. It turns out to be

Katherine Smith Diehl
Hymns and tunes: an index
Scarecrow Press, New York and London, 1966
lv+1185 pages
NLS shelfmark Mus.51.28 (not much use to you, they didn't give it an LC classification)

It indexes an unbelievable number of books. Unfortunately it *doesn't* have a theme code index so you need additional help if the name of a tune changes - but hymns aren't as bad as folk tunes that way.

I used the copy at Edinburgh Public Library. There are a lot of other indexes and histories of hymns out there - do a subject search at your favourite library's online catalogue.