Book 5 in the SF/F series Children of The Dragon by T. M. Moore
In 1878 Robert St. John enlists in the British army to serve in Afghanistan. On the way there he meets two strangers who change his life forever. He becomes a hero, is knighted and recruited by Disraeli to form Her Majesty's Secret Service. He is menaced by Count Vladimir Drakulya, who wants the information he keeps secret. Robert loses everything but his life, but finds a new purpose when he reunites with an old friend to help rid London of the evil haunting Whitechapel Road.
Be careful what you wish for. You may get it.