If you want to sail your FW kit in a little more wind and choppier conditions, my tip would be to buy a more recent FW board - ie, something built after 2004. The really old (longer, more flatter shapes of <2004) are a lot more difficult to sail in bumpy conditions. Each year the boards have become considerably more user friendly; but you should be able to find a 2-3 yr old FW board for peanuts these days...
If you get a more recent board, I wouldn't go below an 8.0m sail. An 8.5m is fine as a medium wind sail and with the extra tail width in these boards you'll be suprised how easy it is to handle such a big rig in decent wind. Use a minimum 56cm fin and you'll be enjoying how much FW improves all your sailing (even when you go back to shortboards) in no time
In regards to books. There are none. There is one decent video around called "Pro Secrets" (google it) however its not as indepth with regards to technique (ie, stance on the board). I have a website you can read some articles all about FW sailing:
Go to the Archives page and start from the bottom up, as most of the newer articles relate to knowledge written in previous articles. There should be enough to get you started there (technique, tuning, starts, tactics etc etc).