Larousse gives 2 definitions of "étagère":
Tablette fixée horizontalement à un support vertical. ex : Les étagères d'une bibliothèque.
Meuble formé d'un ensemble de tablettes superposées.
It seems to me that the first meaning translates to English as "shelf" and the second meaning translates to English as "rack" or "shelving unit". Word Reference agrees with that. I guess that an "étagère à livres" is a synonym of "bibliothèque" (bookcase). Is that right? Is there any difference in meaning between both?