GET request of more than one resource I'm working with services 7.x-3.12, I need to create a web service in order to get a kind of "associative request" between : books(table stored in db) and categories(another table) those two tables are linked by entity_id

I can GET books (retreive, index) same for categories, but how to get Book A -> categorie A ? or the book list of category A

How can I make it work (the get service)?
P.S. book is a content type (node), categories are taxonomy(taxonomy_term)
The short answer is, you can't. I have an issue posted to hopefully make this, and other filters, a reality soon.

In the mean time, you can however create a custom service resource that uses POST, and from there build a custom callback function to return your desired results.

Alternatively, you can use the Views JSON module (a sub module of Views datasource) or Services Views to easily create a View (with contextual filters, normal filters, sorts, etc) that returns JSON data to you via GET.