His first novel is Mars Landing (I have not read it myself). He lists his SF bibliography on his website [1]. It consists mostly of short stories which appeared in various magazines, but a couple of stories are available free on the web [2], [3].

[1] http://www.geoffreylandis.com/G_Bibliography.html

[2] http://www.geoffreylandis.com/time_prime.htp

[3] http://www.geoffreylandis.com/interlude.htp