Harbinger is the first novel in the Star Trek: Vanguard series concerning the Starbase 47, otherwise known as Vanguard.
The U.S.S. Enterprise is damaged after traveling to the edge of the galaxy. They are passing through the Taurus Reach and are surprised to find a new Federation facility, Starbase 47 a.k.a. Vanguard. Captain James T. Kirk is puzzled at this place for many reasons, including it being so near the xenophobic Tholian Assembly. Kirk has his ship dock for repairs.
The Tholian Assembly, the Orion Syndicate, and the Klingon Empire all believe there is much more to the establishment of this odd new starbase and there is.