You searched for: “biblioclept
biblioklept, biblioclept (s) (noun); biblioklepts, biblioclepts (pl)
A book thief or someone who steals books: Besides the direct biblioklept there is the indirect thief who borrows and never returns books to their proper owners (including a library).

Pointing to a page about a kleptomaniac Here is a news article about a bibliokleptomaniac.