But please note https://stackoverflow.com/questions/23727768/which-parts-of-...

"It's old. RWH was written at a time version 6.8 of GHC was being used."

and

"That being said, it's still a useful resource for general guidelines. But keep in mind that many libraries changed since its release."

Only in an 'online' format though. If you want the ePub / PDF then its better to buy the book. Having the content free online is really useful for linking/referencing to the content.

It's similar with Learn You A Haskell [0]. That too is free online, but I read through an ePub copy that I got via a Humble Bundle. I could never actually fully read through an online version of a book I think.

[0]: http://learnyouahaskell.com/

Fair point. It is free in HTML format on its page. And as the authors said (on the page you linked) "We make the content freely available online. If you like it, please buy a copy."

This looks like a trend: to provide a free on-line copy and still sell "better" formats.