Is it acceptable that a teacher assistant encourage students to read his authored materials too?
Your role as a teaching assistant is to further the educational goals of the course (as determined by the instructor and/or the department). 

If encouraging your students to read these materials furthers the educational goals of the course, then you should do so. If not, then you shouldn't.

Since the instructor and/or the department determines what the educational goals of the class are, you should check with them first, and use your extra material only with their approval. (Besides for the issue of making sure your material is consistent with the goals of the course, it also makes sense for them to be aware of what is going on.)