also: ふくどくほん
noun
supplementary reader; supplementary textbook
An additional book used alongside a main textbook, often in school settings, to provide extra material or practice.
授業では教科書のほかに副読本も使います。
In class, we use a supplementary reader in addition to the textbook.
この副読本には練習問題がたくさん載っている。
This supplementary textbook contains many practice exercises.
Compound of 副 (ふく, 'auxiliary, supplementary') and 読本 (とくほん, 'reader, reading book'). The reading とくほん for 読本 is a somewhat older or formal term for a reading book.