If you only say “étagère”, this can be a shelf or a shelving unit, that’s what WordReference tells you. If you say “étagère à livres”, you’re more precise, and it indeed has the same meaning as “bibliothèque”, but “étagère à livres” is never used (at least, after 18 years, I’ve never heard it).
Be aware however of “étagère de livres” which means shelf of book. So, a “bibliothèque” is composed of “étagères de livres”. I hope you will not be confused.