Wirth's Compiler Construction
PLT Online has added a new text to the library: Niklaus Wirth's Compiler Construction. This is a remarkably short book, yet it covers the essentials of building a compiler, from lexical analysis to code generation. The source and target languages used are now pretty obscure (though they should look familiar to anyone who's seen Pascal), but the great thing about theory is that it transcends implementation details. This would be a good text for a self-study student to use as an introduction to compiling.