index: book links

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

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@@ -237,15 +237,16 @@ It's not necessary to purchase these for the class. But they are good ways to ge
 
 *      *An Introduction to Lambda Calculi for Computer Scientists*, by Chris
 Hankin, currently $17 on
 
 *      *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).
+[Amazon](http://www.amazon.com/dp/0954300653).

 
 *      (Another good book covering the same ground as the Hankin book, but
 more thoroughly, and in a more mathematical style, is *Lambda-Calculus and Combinators:
 
 *      (Another good book covering the same ground as the Hankin book, but
 more thoroughly, and in a more mathematical style, is *Lambda-Calculus and Combinators:

-an Introduction*, by J. Roger Hindley and Jonathan P. Seldin. If you choose to read
+an Introduction*, by J. Roger Hindley and Jonathan P. Seldin, currently $52 on [Amazon](http://www.amazon.com/dp/0521898854). If you choose to read

 both the Hankin book and this book, you'll notice the authors made some different
 terminological/notational choices. At first, this makes comprehension slightly slower,
 but in the long run it's helpful because it makes the arbitrariness of those choices more salient.)
 
 both the Hankin book and this book, you'll notice the authors made some different
 terminological/notational choices. At first, this makes comprehension slightly slower,
 but in the long run it's helpful because it makes the arbitrariness of those choices more salient.)
 

+*      (Another good book, covering some of the same ground as the previous two, but also delving much deeper into typed lambda calculi, is *Types and Programming Languages*, by Benjamin Pierce, currently $61 on [Amazon](http://www.amazon.com/dp/0262162091).)

 
 *   *The Little Schemer, Fourth Edition*, by Daniel P. Friedman and Matthias
 Felleisen, currently $23 on [Amazon](http://www.amazon.com/exec/obidos/ASIN/0262560992).
 
 *   *The Little Schemer, Fourth Edition*, by Daniel P. Friedman and Matthias
 Felleisen, currently $23 on [Amazon](http://www.amazon.com/exec/obidos/ASIN/0262560992).