The author defines Legacy code to be any code not covered by unit tests (which is rather strict).
He also insists that individual unit tests that take 0.1 seconds are slow.
This prevents unit tests from relying upon databases.
I have a slight problem with implementing that – the main projects that I work on have a fair amount
of stored procedure code. These need unit test coverage (plus the interfaces with the rest of the code also needs to be tested).
The book also provides strategies to get Legacy code under test.
Rober C Martin’s Agile Software Development : Principles, Patterns and Practices (Pearson Education, 2002).