Working Effectively with Legacy Code is a book that should be a mandatory reading for anyone who is interested in coding for a living. I don't deal with many legacy systems at the present (except for NQA, of course :-) ) and still this book has been a fountain of knowledge about techniques, tips and tricks.

This is not a book about pretty code, it's a book about ugly code and all the myrad ways you can abuse design pricipals and good coding practices. What it does is show a set to systematically take a legacy code* and break down the dependencies so you can write tests for the code. The moment you've tests, you can move forward. Until you've tests, you're in a position where all you can do is "Edit & Pray" or... you can write tests.

I've had recent experiance with developing applications in a TDD from the start (Rhino Mocks) and converting a legacy application to TDD (NQA). I can't rave enough about how much easier it is to make a change in Rhino Mocks because of those tests. When I adapted Rhino Mocks to run on .Net 2.0 I'd broken just about every part of the application because I didn't consider what the change from Hashtable to Dictionary<T> would do when there is no item in the dictionary. Because I'd tests, it was literaly the work of a minute to find the problem and fix it.

One thing to note about this book, it took me over two months to read it. Like Xp Explained 2nd Ed. I just can't seem to read it for any length of time without starting to feel a sudden need to code something. Very productive for my projects, less so for my reading backlog.

* Legacy Code == Code without tests.