Yo fam, the subject in this sentence is "librarian", 'cause that's who we talkin' about, and the main verb is "found" 'cause that's what the librarian was up to – findin' a book while putting others back on the shelves. So, put it together and it's "librarian found". It's like when you're scrolling through your feed and you spot that dank meme - you're the one doin' the scrolling, so you’re the subject, and "spot" is what you're doin', that’s the verb. Same energy.