Soshite Haru no Tsuki

In the turbulent final years of the Tokugawa Shongunate, Shieikan Dojo was the birthplace of one of Japan's most famous militias: the Shinsengumi. Many of the members have acheived legendary status for their skill, honor and devotion. Under the banner of "loyalty", they would sacrifice their lives for their ideals...

"Soshite Haru no Tsuki" is a series by Miya Ikushima, currently spanning 4 books that give a decidedly yaoi-friendly interpretation of the Shinsengumi. The main pairing is between the young first captain, Okita Souji, and his vice-commander, Hijikata Toshizou, but practically all of the well-known members have their own stories... and plenty of steamy love scenes interspersed with the fighting and intrigue the Shinsengumi were known for. (from hanami)

The books of the series are:

Vol.1 Soshite Haru no Tsuki (1998)

Vol.2 Akeyuku Sora (1999)

Vol.3 Yakusoku no Oka (2001)

Vol.4 Chiranai Hana (2002)

Vol.5 Ten to Chi to Kaze to (2006)