I am open to recommendations as to:
What textbook would be good?
What compiler(s) are available in Ada for modification?
Definately take a look at Crafting a Compiler, by Fisher & LeBlanc.
They actually have as a running example building a language that is a
subset of Ada 83.
(Note that there's another, similarly named book, called Crafting a
Compiler in C. The authors re-wrote the examples in C, instead of the
Ada-like syntax of the earlier version.)
There are a couple of other books aren't as heavy on the theory (a good
thing, methinks). Check out:
Compiler Design in C
Allen I. Holub
Writing Compilers and Interpreters
I couldn't really make sense of the Dragon book. I have no idea why
that book is so popular.
To modify a compiler, you could use GNAT. The sources are freely
available from NYU.
I know you can buy the source in the Holub book from the author (US$80
last time I inquired).