Man, I'm surprised only one person recommended Pressfield. Of course, though, it would be Furius. Bravo, exemplar of the Furii.
I'd also recommend:
"The Western Way of War" by Victor David Hanson (an incredibly vivid, insightful manifestation of what a hoplite battle actually entailed from the march out to the aftermath. Of course, such detail is conjecture, but you'll be hard pressed to find a better source).
"A War Like No Other" also by VDH (his recounting of the Peloponnesian War).
"Warfare in the Classical World" by John Gibson Warry and John Warry (Much like "Greece and Rome at War", this large book is useful mainly for its pictures and battle diagrams).
"Greek and Macedonian Art of War" by Frank E. Adcock (An oldy but goldy, this thin pocket book is a compilation of insightful lectures on everything from siege warfare to naval combat. Light but useful).
As for fiction, well, I'm biased. I confess the only author ever to hold my attention was Steven Pressfield. No one else comes close in my opinion. Aside from the obvious one, "Gates of Fire", I particularly loved the following:
"Tides of War" by Steven Pressfield (details the rise and fall of Alcibiades in the Peloponnesian War as told through the eyes of the mercenary who killed him. This one's a bit noirish for most people's tastes because there are no real heroes in the story. But that's why I loved it; everyone was real, tangible, and human. The highlight of the work is Pressfield's description of the disaster at Syracuse).
"The Virtues of War" by Steven Pressfield (first hand narration of Alexander the Great's exploits as transcribed by one of his captains. Pressfield does the impossible by painting Alexander as a truly noble--if flawed--hero. The writing is vivid, succinct, and intuitive. A must have for any Alexander-philes out there).
I'd recommend Pressfield's "Last of the Amazons" for its description of that fabled female warrior horse-culture, but the story is a bit weak in my mind. Still, if you're interested in the Amazons, it's an eye-opener).
"All right, but apart from the sanitation, medicine, education, wine, public order, irrigation, roads, the fresh water system and public health, what have the Romans ever done for us?"
"Sometimes, a view from sinless eyes,
Centers our perspective and pacifies our cries..."