In his book Why Should Anyone Believe Anything At All, James Sire recommends the following books on The Historical Reliability of the Gospels:
Is the New Testament Reliable?, by Paul Barnett
The Historical Reliability of the Gospels, by Craig Blomberg
The Evidence for Jesus, by James D. G. Dunn
The Evidence for Jesus, by R. T. France
I Believe in the Historical Jesus, by I. Howard Marshall
Christ and the Bible, by John Wenham