Congrats on your move to MarkLogic!

Bear in mind that if you use $book/bk:title/text() you’ll run into problems with mixed content (that’s why we don’t do that in XSLT). If that’s something you need to worry about, you probably want to pass the bk:title to a function that handles its content properly.

If bk:title doesn’t have mixed content, or if you don’t care about the mixed content, I’d be inclined to use data($book/bk:title) or $book/bk:title/data(.) if you prefer, which returns an atomic value, rather than $book/bk:title/text(), which returns a sequence of nodes. But I think this is because of years of suffering with first-item semantics in XSLT.