Bob borrowed a book from the library and months later, accidentally dropped his bag (with the book in it) in a river. He retrieves the bag, but the book is ruined, being all sandy and wet. What is the consequence of this accident legally speaking?
If this is a public library in England or Wales, then they are allowed to charge for lost or damaged items, at their discretion, even though there is a general duty for libraries to make their normal lending services available free of charge.
The Library Charges (England and Wales) Regulations 1991, a statutory instrument made under section 8 of the Public Libraries and Museums Act 1964 of says this in regulation 3(2)(e):
A relevant authority may make a charge [...] in respect of library apparatus, library material and any other equipment or thing used in providing the library service which is lost, damaged or destroyed by, or whilst on loan to, the person paying the charge.
Similarly, they can charge for late return of items, and for various special services. "Library material" includes "words, figures, images, sounds or data recorded in or on any medium", which certainly covers books - even picture books. The library has broad discretion as to the amount and terms of any charge, but the replacement cost of the lost item is a typical starting point.
Whether it was Bob's fault that he dropped the book in the river is irrelevant. He is on the hook for the charge. Potentially, he could claim against somebody else if they had damaged the book and thus caused him to suffer a financial loss, but that does not affect the fact that Bob has to pay the library.
While other libraries could make this part of their contractual terms for using the service, this specific regulatory provision is necessary because the default position for public libraries is that they cannot charge any fees to local residents for borrowing books; this aspect of their operations is a matter of public law, as opposed to a contract between the library and any given local resident.