Site etiquette dictates that one should refrain from posting an answer that has already been given, so perhaps I shouldn't be posting this. However, I think there is a point that has not been explicitly addressed but that needs to be made.

The word for "tejuelo" is English is "label", without doubt, but it is applied to two different kinds of label. One is the kind you've described in your question, the kind librarians attach to the spine of a book, generally near the bottom, to indicate its place in the library's classification system and its physical storage position on the shelves. The other is part of the binding of a book, a piece of leather, cloth or paper also attached to the spine, but normally in the upper half, bearing the title and often the author's name. Obviously modern books don't usually have such labels; cloth-bound hardback books usually have the author's name and title tooled on the spine, and of course these details are printed on paperbacks and dust covers. But leather-bound books commonly do have them.

The RAE definition speaks of the "tejuelo" bearing the "rótulo", and this can and does apply to either of the cases mentioned.

This second type of "tejuelo", the kind with the title and author's name, is called a spine label or backstrip label, or just label if the context makes it clear this is what the word refers to. Strictly speaking the spine is the "backbone" of the book, sewn and/or glued and not visible (unless the binding is damaged), and the visible covering of the spine is the backstrip, but this distinction is often not applied, and "spine" is commonly used (including, for example, by the British Library) to mean both. So "spine label" is fine, as well as "backstrip label".

"The binding of our new Capreolus is in seventeenth-century green vellum over pasteboard, adorned only by a brown leather spine label, now fragmentary (CAPREOL. SENT, can be read)"
http://www.bl.uk/eblj/1994articles/pdf/article7.pdf

I can't find any references to "backstrip labels" in the BL's own book descriptions, but plenty of bibliographers and booksellers do use the term:

" BACKSTRIP :
The covering of a book's spine, usually of cloth or leather, and frequently lettered with the title of the book and other information, and decorated as well. The backstrip will often show signs of wear before the rest of the book, fading, fraying, becoming chipped or discoloured. Often SPINE is used to include the backstrip and spine together.
BACKSTRIP LABEL:
A label printed with the title of the book, sometimes also the name of the author, which is affixed to the backstrip. The label may be leather or paper. In either case, this type of label, which adds to the cost of manufacturing the book, is rarely used on modern books except as a special finishing touch on fine and limited editions."
http://www.sageold.com.au/terms.shtml

Well, so much for type 2. Now, back to type 1, the kind of "tejuelo" you actually asked about. This is most often, and I think properly, called a shelfmark label, for the simple reason that the proper term for a book's classification reference, its "signatura", is shelfmark (or more rarely classmark). I can positively say this is so in the UK, on the basis of long experience in research libraries, and I believe it is so in other English-speaking countries.

"Large books
Each floor has some books that are too big to fit on the normal shelves. These books have the word Large at the beginning of the shelfmark and an 'L' in front of the number on the shelfmark label on the spine of the book, e.g. L724.5/JEF"
http://www.port.ac.uk/library/help/skills/glossary/

"The cypher of King George III is tooled in gold on the flat spine but is now hidden by the shelfmark label."
http://www.bl.uk/treasures/shakespeare/henry4p2bibs.html

"Nineteenth-century half calf, shelfmark label on spine"
http://www.sothebys.com/en/auctions/ecatalogue/2008/property...

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Note added at 9 hrs (2016-05-21 00:07:21 GMT)
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Here's a very lengthy Spanish blog about the word "tejuelo", showing that it applies to both kinds; most of it is about the spine/backstrip label kind, but shelfmark labels are also covered lower down.
http://palabraria.blogspot.com.es/2013/02/tejuelo.html

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Note added at 9 hrs (2016-05-21 00:21:25 GMT)
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Just to be clear, for the question you actually asked I don't think either of the previous answers is sufficient. "Spine label" doesn't mean a library label (no competent librarian or bibliographer would understand it to mean that), and "label" alone could refer to a number of things and by default would usually be taken to be a title/author label. If you want to refer specifically to a library "tejuelo" I think you should call it a shelfmark label.