Maybe the book was retrenched.
I know it is not encouraged to be speculative at ELU, but because real facts are lacking, I may be allowed to take up the context of somebody's comment elsewhere, referenced in the answer by @ab2 here, that 'this book was drenched' has something to do with getting rid of the physical copy after the book is digitised, as in
this message shows up on about 40-50 books
which were digitized (...) and means that immediately after digitizing the book they purged the physical book from their collection
and this part of the very next comment on the same page (in reply) by another user:
Well, library space costs money and they're getting smaller and
On this basis, and especially because the comment seems to refer to the library of an Indian university, I shall hypothesise that 'drenched' here could be an unintended phonetic misspelling of retrenched which is defined as follows by Merriam-Webster dictionary online:
retrenchment [noun] : reduction, curtailment; specifically : a cutting of expenses
The books were retrenched to curtail the expense of storing the physical copies:
This book was retrenched.
This type of spelling error can occur when the person writing it knows neither the word nor its spelling but is simply rendering a word heard spoken by somebody. If I or another Indian were to casually say 'retrenched', the 're' could be weak and there might be more emphasis on the second syllable (trenched), or maybe a lorry passed by outside with a heavy grinding of gears that obscured the 're' sound -- moreover some Indians tend to pronounce the 't' sound in the middle of words as a 'd' sound -- somebody else could easily hear and render it phonetically in writing as 'drenched.'
Of course that person would not know the word 'drenched' or its meaning either.
Head librarian says: when a book is scanned we need it to say on the first page, "the book was retrenched."
The assistant hears an unknown word and phonetically types out another unknown word: the book was drenched.
On the other hand, an early step in the recycling of paper is to soak it in water for a few days...
This book is going to be recycled. This book was drenched.
This is much less likely because Indian universities are bureaucratically very serious places without any sense of wit or humor. A University is a 'temple of learning' where students come to acquire Knowledge for a better life -- this is no place for subtle linguistic wit and riddles -- keep your humor outside the Library Door! So I leave you with Dante's immortal quote until we can get a more factual answer.
Important note to would-be editors of my answer: since I should prefer to be subtle once in a while, please do not find, copy and paste-in Dante's immortal quote referred to ironically here. Let interested readers follow the link and learn more.