There might be a difference in meaning, since "Unterrichtsbuch" seems to designated for actual classes in a school and "Lehrbuch" seems to be a more generic word. E.g. a "Lehrbuch" could be a book, meant to support a teacher, or it could be a book, in which you could study on your own, outside of any school environment.

Still, in the end, for me (native speaker) there is no real difference. Usually the context dictates, what kind of book you are looking at.

There might be a difference in meaning, since "Unterrichtsbuch" seems to be designated for actual classes in a school and "Lehrbuch" seems to be a more generic word. E.g. a "Lehrbuch" could be a book, meant to support a teacher, or it could be a book, in which you could study on your own, outside of any school environment.

Still, in the end, for me (native speaker) there is no real difference. Usually the context dictates, what kind of book you are looking at.