Does reading _Learn You a Haskell_ online make it any less of a book? My point is that "online sources" is a superset of "books", making them incomparable.
No. However a lot of the things I, anecdata, find when I do a google search aren't books, but blogs and random tutorials and stackoverflow.
Books, due to their published nature, tend to have a greater, more thorough, effort put into them. They are generally well laid out, usually well edited, and because people spent money on them the good ones tended to stay in the market and the bad ones tended to drop out.
There's no fundamental difference (except for me, I do much better at recalling things I read from physical books than ebooks), in the source of online versus offline. But it's hard to ignore the difference in quality that tends to happen.
What would be nice, and this is probably done somewhere, would be a reliable aggregator of good online documentation (whether it's an ebook version of a published text or not). Somewhere we can vet and see vetted quality sources of tutorials, manuals, etc.
This is among very few exceptions. Most of the books are still proper books (no matter if they're also available as e-books). Most of the stuff people use for learning online is far below a book quality.
