also: ふくどくほん
supplementary reader; supplementary textbook
An additional book used alongside a main textbook, often in school settings, to provide extra material or practice.
In class, we use a supplementary reader in addition to the textbook.
This supplementary textbook contains many practice exercises.
参考書 is a reference book or study guide, often used for self-study, whereas 副読本 is typically used in a classroom setting to supplement the main textbook.
Standard kanji spelling; the reading ふくとくほん is the most common.
Alternate reading ふくどくほん is less common but recognized.
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.