series of books; library (set of books)
Refers to a planned set of books published under a common theme or title, like a publisher's series or a multi-volume collection.
This series consists of ten volumes.
He collects a series of classical literature.
Standard kanji form; the most common spelling for a series of books.
Alternate kanji form; less common but still used, especially in some publisher names.
From 叢 (collection, gathering) + 書 (book, writing). The kanji 叢 suggests a dense cluster, metaphorically a collection of books.