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