Steel Dreams by Jason Collins

GREY:
Dirty cops don’t clean up well.
Mitch may look like a decent cop, but he’s got issues. I should know—he’s been hot and cold since the day I partnered with him. One minute he’s showing me the ropes, and the next he’s stiff and hard-lipped. Worse, he’s under internal investigation. By me.
I can’t figure him out, but I desperately want to.
Mitch is keeping secrets, but when he kisses me, I know that’s real. Everyone says a bad cop won’t change, and I shouldn’t trust this one. But I’m a detective, and he’s the man I’m after.

MITCH:
Lone wolves don’t need partners.
I follow the letter and the spirit of the law, and that makes me an unpopular cop. Gray is young, impulsive, and aggravating—and now the captain’s partnered him with me. It’s impossible to keep him at arm’s length, and even harder to admit how my body responds to him.
I want him. More than I’ve wanted anyone.
But we can’t be together. I’ve made mistakes I can’t admit to, and I’m breaking department rules every time I bring him home with me. Gray doesn’t trust me, and he shouldn’t—not until I can prove who I really am.

This is the second book in the Brothers in Blue series. It can be read as a standalone with no cliffhanger.