comment on books

Signed-off-by: Jim Pryor <profjim@jimpryor.net>

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 ## Recommended Books ##
 
+It's not necessary to purchase these for the class. But they are good ways to get a more thorough and solid understanding of some of the more basic conceptual tools we'll be using.
+
 *      *An Introduction to Lambda Calculi for Computer Scientists*, by Chris
 Hankin, currently $17 on
 [Amazon](http://www.amazon.com/Introduction-Lambda-Calculi-Computer-Scientists/dp/0954300653).