Book reference in a node

Need help!

I have two types of contents :
- Thématique : content of type book, so that a Thématique has one parent Thématique and can have childs of type Thématique.
- Ressource : a content with a node reference field to a Thématique (so that each Ressource can be attached to one or more Thématiques) and some others fields.

When I create a new Thématique, I can easily choose its parent with Hierarchical Select for Books.
But when I create a new Ressource, for the moment I have a long select list with all the existing Thématiques, which isn't really user-friendly, above all because some Thématiques have the same title...
I would like to have exactly the same display for that field as the one of the Thématique parent field (but with the ability to attach my Ressource to as many Thématiques as I want).

How to do? I can try to modify the code somewhere, but I'm too lost to manage it alone...

Thank you very much!