I personally think that the available documentation (when updated) is great, simple and easy to understand. There are tutorials, examples, and the full documentation well explained, I think it can hardly be better :lol:
Most problems for beginners on using it are usually general programming and architectural issues more than SFML particularly.
Maybe the wiki needs a page with external links pointing on that direction, but usually users aren't interested in exploring concepts and trying things out of SFML, they mostly want to go directly to SFML, put hands on work and try it. Then the perfect would be some documentation involving both things, but that's too much for SFML documentation, it's out of scope. That is where a book could get in, something like this: "Multimedia and Game Programming with SFML", teaching programming techniques and how they apply with SFML.