[eluser]rogierb[/eluser]
Get a book on PHP and Mysql first. CI is build on PHP so it seems only logical to first get a basic understanding of PHP before jumping into CI.
Then again, both can be done at the same time but remember that coding in CI is 'easier' then just plain old PHP.
I would get a decent PHP/SQL book and go from there. A few articles on MVC would be a good idea too.
As for a CI book, the documentation and tutorials normally would be enough, but if you got caps to spare...