There is a new whitepaper written by Brian Knight (pragmaticworks) available. It is a short introduction into SSIS Expressions and can be downloaded for free.

Brian is a trainer for SSIS and here he describes the reason to write this whitepaper:
The idea for this whitepaper came out of teaching countless SSIS classes where students gave us a very confused look when we showed expressions. Frankly, I can’t blame them for the look. The expression language feels like a secret society of confused developers ambushed C# in the back alley after a party, beat it senseless and out came the expression language after some reconstructive surgery. It has some constructs of C and feels like the expression language of Reporting Services.

You can find the download here, registration required!