Glyn, start with a copy of "EZ Guide Route 66" by Jerry McClanahan. It's one of the best guide books, and like the title says very easy to understand. Also, go to your states web sire for guide books, and maps. A week or so is plenty of time for Missouri Route 66. I suggest you not take the section of Route 66 in St Louis as it goes through some quite unsavory sections of town, re: the movie "Vacation." with Chevy Chase :).
Maybe start at the Chain of Rocks in Mitchell, IL, and taking Hwy 44 to Rt. 66 exit in Allenton (If you like you can back track up 66 to the Ted Drewes Frozen Custard area). After that it's smooth sailing with many great sections of Route 66 to drive.

Disclaimer: My suggestion to skip 66 in St. Louis is just for IMHO safety reasons. We have driven the section without problem, but would not do it again after seeing it is just a run down section of the city. If you decide you would like to see that section please do it in the daytime.