• 三冊さんさつ — three books
• 何冊なんさつ — how many books
• 冊数さっすう — number of books
• 冊子さっし — booklet / pamphlet
• 別冊べっさつ — separate volume / supplement
• 合冊がっさつ — combined volume / omnibus

Originally a pictograph of bound slips for writing; now it marks books as units in counters like 一冊・二冊.