The Financial Times article, What drives people to steal precious books, does not spend much time answering the question posed by its title; the article, which references recent map thieves, does talk about how people steal precious books (and maps), as well as the impact on library security. There’s also a brief discussion of inside jobs. As for motive:

In newspaper reports of such crimes, epithets such as “gentlemen thieves” are liberally applied to men such as [Farhad] Hakimzadeh and [William] Jacques. Typically, they are characterised as obsessed academics willing to do almost anything to obtain that ancient tome or map that will fill a gap on their bookshelves. Hakimzadeh’s defence revealed that he spent his wedding night polishing his beloved books, while Gosse offered his own love of books as mitigation for his crime. “I felt the books had been abandoned,” he said. He was given a suspended sentence, a €17,000 fine and was allowed to go back to his teaching job. The archbishop forgave the thief and said he would even allow him (supervised) access to the library.

Via Map the Universe.

Previously: “A Notorious Gentleman Thief”; Hakimzadeh Sentenced to Two Years.