reference book; study guide; supplementary textbook
A book used alongside a main textbook or for self-study, often containing explanations, practice problems, or reference material. Common in educational contexts.
I bought a study guide and studied before the exam.
This reference book has easy-to-understand grammar explanations.
I looked up a word I didn't know in a reference book.
I'm using this reference book to study for the entrance exam.
That formula is in the reference book.
I bought a new reference book at the bookstore.