Lovelace, an interactive Ada tutorial
Ada 95: A guide for C and C++ programmers (it's not really descriptive enough to be useful)
The Big Online Book of Linux Ada Programming (it doesn't describe Ada programs until Chapter 10!)
Ada information clearing house, an advocacy page. I recommend its ``Ada at Work'' section.
Although I haven't done a thorough book search, I did find Gary Bray and David Pokrass's book Understanding Ada: A Software Engineering Approach (Krieger Publishing Company) to be competent - the only on-line work that I found of comparable quality was Lovelace. It costs $42.50 through Amazon.