God or Godless? by John Loftus and Randal Rauser is $0.99. Each chapter is a short discussion of a particular topic in the style of formal debate (opening statement, rebuttal, closing statement). Loftus is an atheist and Rauser is a Christian apologist.
Even though Loftus competently argues his side, the topics are varied enough to allow Rauser to address a lot of theology that isn't normally discussed. I'm pretty sure that Rauser's thoughtful responses are what prompted Baker to publish the book.
eBooks.com
Kobo
B&N
Christianbook (where it's $0.79)
Amazon