[PRL] reading list for theoretical topics in PL

[Dimitris Vardoulakis]

Hi all,
one could say that the following topics:

lambda calculus, type systems, operational/denotational semantics, 
universal algebra, logical relations, category theory

(that loosely correspond to the chapters of J.Mitchell's book 
"Foundations for Programming Languages") should be studied by an 
advanced undergrad/beginning grad interested in PL. There are naturally 
many books (by Winskel, Gunter, Pierce, Mitchell etc) that cover 
some/all of these topics.

I'd be interested in free, good sources to read about these topics. For 
example, I think that these two papers are a good place to start reading 
about type systems:
- L.Cardelli, "Type Systems", the handbook of CS and Eng., 1997
- L.Cardelli, P.Wegner, "On Understanding Types, Data Abstraction, and 
Polymorphism", ACM Computing Surveys, 1985
For lambda calculus there's the classic chapter by Barendregt.

Similarly, I'd like to know:
a) good sources for the rest of the above topics
b) topics missing from the list? topics in the list that are not needed?

thanks in advance,
Dimitris