Here are some resources that have been useful for me when it comes to selecting spanish books.
americareadsspanish.org – Has book reviews in Spanish and English. Shows newest releases from all over latin america as well as best sellers.
Lectorum– vendor of spanish books
Latin American Book Source– vendor of spanish books
megustaleer.com – Penguin Random House Grupo Editorial website that covers new releases, reviews and publishing trends.
Where you can find bilingual and spanish books -Bookriot article- Great article for selectors just starting out.
Spanish Selecting- Collection Development from Library Journal – a quick read for just starting out with your Spanish collection.
LA Librería- Books for Kids en Español
Checking in with publishing companies is always a good idea, as is attending local book fairs such as your local REFORMA book fair or even FIL if your library can manage that.