From the Torah Musings site(https://www.torahmusings.com/2011/05/sefarim-part-i-of-ii/), on footnote #20 he writes:
One who borrows a sefer and ruins it or wears it out due only to extensive usage is exempt from paying any damages.
The sources being: Minchat Elazar 4:70 cited in Minhag Yisrael Torah 14:2.
Is a light stain from food or drink considered extensive usage (i.e the person was accustomed to eat and/or drink while learning)?
Note: the book is still legible/readable.
Answer from Horav Y. Fleishman shlit”a
I don’t really know what they found new in Minchas Elozor since it’s a Gemoro that damage due to extensive use if it was to be expected is called meiso machmas melocho. However,it is not assumed that the borrower will eat over the sefer unless it was stipulated. Therefore, the borrower will need to pay for the damages