As mentioned above, our Deals forum on MobileRead does have quite a few Christian books that are offered for free, so it is worthwhile to check daily plus following the Books on the Knob blog.
I used the
literature map to find suggested similar authors. It shows that Lynn Austin is close to Deanne Gist, Kristen Heitzmann, Denise Hunter, Jeanette Oke, and more. It is generated from suggestions by readers, so it is not perfect, but does have some good authors to start checking.