I thought I bookmarked this a long time ago – apparently I didn’t.
O’Reilly’s Test-Driven Development with Python (by Harry Percival) is still available to read online at the O’Reilly Atlas site (formerly O’Reilly’s Chimera Labs, if I remember correctly and if the URL is any guide).
Percival’s book was made available to read for free courtesy of O’Reilly’s Open Source Convention (a.k.a. OSCON). Since my budget is currently limited, I’m very happy for the opportunity to read this book without having to buy it (or keep checking it out of the library).