I find several advantages to well-written books than to online docs:

You can annotate books. It's a lot harder with on-line docs.

It's easier (for me, at least) to see the surrounding context when I'm looking at a bound book, as opposed to online docs. The surrounding context can be essential for true understanding

Your desktop usually has more space than your screen. Having several books open on your desktop for quick reference is easier than having several online docs open on your screen at one time.

etc.