Sixty-One Nails, by Mike Shevdon, is the first in his Courts of the Feyre series.
Book Description
THERE IS A SECRET WAR GROWING BENEATH THE STREETS OF LONDON.
A dark magic will be unleashed by the Untainted...Unless a new hero can be found. The smarter, faster brother to Neil Gaiman's Neverwhere has arrived.
The immense Sixty-One Nails follows Niall Petersen, victim of a suspected heart attack on the London Underground, into the hidden world of the Feyre, an uncanny place of legend that lurks just beyond the surface of everyday life. The Untainted, the darkest of the Seven Courts, have made their play for power, and unless Niall can recreate the ritual of the Sixty-One Nails, their dark dominion will enslave all of the Feyre, and all of humankind too.