I think it's a great sign that the teacher called, and definitely that opens a door to a follow-up email. You could let her know that after the call, you did start thinking of a few concerns based on some things your son told you.
I would start by picking one thing that you think your son is most upset about. If it's the picture books, maybe say "My son was pretty disappointed that he was limited in his choices of books in the library/in class. I understand that this early on you probably are more focused with getting the kids used to the different procedures in school, but I wonder if it would be OK for me to send in a few books for him to have at school?"
