As I recall:
- Interplanetary war featuring bipedal mechs piloted by disembodied brains.
- The "good guys" used mortally wounded regular soldiers to become mech pilots.
- The "bad guys" killed their own volunteers in an elaborate ceremony before turning them into mech pilots.
- Many of the pilots had psychological problems due to their situations.
- At one point a female officer on the "good guys" retires to her spaceship's quarters to view prisoner torture and interrogation on the planet below via robot or surveillance system and pleasures herself (bizarrely memorable, obviously)
- The story was published sometime before summer 1999, because I gave the mass-market paperback version to an acquaintance at that time and haven't seen the person since, so the date is clear in my memory.
- It evidently became a series, as I recall that acquaintance later writing me to say he had read several more books in the same series.
Best of luck and many upvotes to the finder! :-)