From en.wiktionary.org:
** English
*** Etymology
From [en].
*** Noun
[en-noun]
1. The vertical surface forming the back of a shelf.
2. * {{ quote-book | en | year=2001 | author=Regina McBride | title=The Nature of Water and Air | page=62
|passage=She came home later that day from Mrs. Rafferty's with a blue
platter, which she placed standing up against the SHELFBACK in the linen
closet.}}
1. The spine of a book .
2. * {{ quote-journal | en | year=1934 | journal=The Atlantic Monthly | volume=153 | page=71
|passage=There was the man who collected any old volumes on etymology. There
was a collector of books with gilded Victorian SHELFBACKS.}}