Responsa for Bava Metzia 161:4

Q. A borrowed a book from B for one hour with the understanding that after the hour B would send for it, and if not, he should bring the book back to B. A few hours later a fire broke out in A's house, as a result of which, A was compelled to flee for his life, as the Gentiles were accustomed to throw into the fire Jews in whose houses a fire broke out. A could not, therefore, save B's book.
A. After the period, for which A borrowed the book, had passed, A's responsibility for the book became that of a hired watchman. Therefore, if, before he fled, A could have hired somebody to save the book, and did not do so, he was liable for its loss. But if this was impossible, he was free from obligation.
SOURCES: Pr. 140; Mord. B. M. 376; Agudah B. M. 152.

Q. C and D lent books to A (of the Responsum given above) with the understanding that A, in turn, lend books to them, which he did. C and D's books were burned during the fire in A's house.
A. Since there was a mutual exchange of books, A's responsibility was that of a paid watchman. Therefore, the answer given above (no. 743) is applicable to this case also.
SOURCES: Pr. 141; Mord. B. M. 376; Agudah B. M. 152.

Q. A demanded from B the book of the order Kodashim which he lent him to copy from. B, in turn, demanded from A the book of the order Moed which he gave to A as security, and which was worth twice as much as A's book. At first A denied that he received B's book, but when confronted by a witness who supported B's claim, A retracted his denial, but alleged: 1) that B loaned him the book with the understanding that his (A's) children should be permitted to study therefrom; 2) that he loaned the book to his son-in-law; and 3) that the book was subsequently burned in the latter's house, and that hence the loss of the book was an unavoidable accident. B denied that he gave A permission to allow his sons to use the book.
A. Since A and B lent books to each other, their responsibilities regarding these books were those of hired watchmen who are not responsible for unavoidable accidents. The question revolves, then, about A's lending the book to his son-in-law. Therefore, upon A's taking an oath that B gave him permission to allow his sons to use B's book, A will be free from any obligation to B. B, on the other hand, may retain A's book, upon taking an oath that he did not permit A to give his book to A's sons for study, and B is further entitled to collect from A's son-in-law (the difference between the value of the lost book and the one B retained) since the responsibility of the son-in-law regarding the book was that of a borrower who is responsible even for unavoidable accidents. If, however, A's son-in-law is willing to take an oath that he did not borrow B's book from A, he will be free from obligation to B, and his oath will not subject his father-in-law to any further obligation.
The question was submitted by R. Moses Azriel b. R. Eliezer Darshan, whose opinion, appended to the question, coincided with that of R. Meir.
SOURCES: Pr. 963; Mord. B. M. 282. Cf. Isserlein Pesakim 200.