S (Esu): series
Title rating: 18+ only for graphic prose sex scenes and a few graphic illustrations, violence, language, some torture, etc.
- 4 novels (2005-2006), in English via DMP, complete
- 4 Drama CDs (2005-2007), complete
- 2 spin-off Drama CDs (“I’m Not Your Pet” and “I Swear”) (2007-2008), complete?
- Various spin-offs and doujinshi novels (ongoing)
There is an amazing review of these four books and the cds and such
here. I've read the first 3 books and am starting on the fourth so I'll leave it to someone who has read them all this time.
For the uninitiated though:

S Vol. 1
Shiba is a detective in the fifth anti-organized crime division of the Tokyo Metropolitan Police, a.k.a. "Anti-Gang 5" specializing in handgun seizures. Shiba's secret weapon is "S" ("spy"), an informant who plays a key role in Shiba's information gathering. Shiba is training an informant who has been officially accepted into Shinjuku's Matsukura Group within the Budoha crime syndicate. One day, Shiba awakes to an anonymous phone call. "Watch out for your S." A tale of two men whose lust, pride and loneliness collide!

S Vol. 2: A Love Bite
Detective Masaki Shiiba’s informant or “S,” Keigo Munechika, has proven himself to be an excellent source of information. But the payment of sex for every lead given is taking its toll on Shiiba. But with his new assignment as back-up to a fellow undercover detective, Shiiba learns that there are far worse things than having sex with your male S—falling helplessly in love with him, for example. S volume 2 continues the drama and passion between a detective and his spy set in the highly-charged underground world of Tokyo.

S Vol. 3: Split
For almost a year now, Shiiba has been using Keigo Munechika as his “S,” his spy, to get information on the gun dealings of the Yakuza world. Shiiba uses his body in exchange for Munechika’s sensitive information... and only his body as far as Shiiba is concerned. Shiiba vows their relationship will not go beyond that. But when a rowdy young man with a penchant for piercing and an enigmatic Yakuza leader cross paths with the young detective, sins of the past are unearthed once more-sins that just might shatter the fragile bond between the detective and his “S.”

S Vol. 4: Afterglow
At the end of S volume 3, detective Masaki Shiiba was last seen running away from the hospital room of his injured S, Keigo Munechika. Now, Shiiba has turned in his resignation, bought a gun of his own, and is bent on only one mission—to kill Takanari Godou. He vows to get the truth from Godou, even if he has to throw away his morals and life to do it. But can he do it? Can he shoot another person and commit cold-blooded murder for revenge?
A part a highly agree with is:
"S does have its own weak points, ones that are particularly obvious to fans of non-yaoi crime drama. The writing is definitely a bit stilted and simplistic compared to a Western novel, but that’s standard within yaoi novels that don’t feature an extremely dedicated/slightly masochistic rewriter. S definitely isn’t the worst prose adaptation out there, and the story is so intense that it’s easy to overlook a poor writing style in a fervent desire to find out what happens next."
This is probably my biggest complaint with Yaoi novels. They always feel like a very young writer has written them and while I completely understand that this is a translation and the editors are trying to keep as close to the original as possible, I'm still unhappy that some of my LJ fanfiction friends have better control of story plot and development.

Another of her books is
Decoy, which apparently has cameos of the S characters and takes place in their world.
Deadlock is another, though unrelated, work:
Yuuto Lenix, a former DEA investigator was arrested by a false accusation of killing his own partner. The FBI agents gave him a special deal, to look for a particular criminal hiding in the same prison. By the time he was detained in the Schelger Prison, he got to meet his cellmate, Dick Burnford, who didn't show signs of being kind and welcoming upon his arrival. Will he ever be in good terms with his inmates, especially to Dick?
Volume 1: Deadlock
Volume 2: Deadheat
Volume 3: Deadshot
Deadlock Side Story - Tooiyoake
Deadlock Side Story - Tsuyoki Mono, Nanji no Na wa Onna
DEADLOCK Side Story - Love begets love