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
Ronald Mak

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).