"This book is a contribution to the history of Palestinian Jewish Christianity in the New Testament Period."
At least a subtitle such as "A Contribution to the History of Jewish Christianity in the New Testament" would have revealed the central importance of this work.
Only recently have I had occasion to read parts of the book and I am again, as I have so many times by Bauckham's work, struck by the clarity of his conclusions based on detailed research of primary source material.
In the book he studies Jewish Christianity in the NT by considering three topics: (1) the significance of Jesus' family (excluding James) in the early church, (2) the book of Jude and (3) Luke's genealogy. While it is not apparent at least at first how the third point relates to the first two, Bauckham sees Luke's genealogy as offering evidence--along with the letter of Jude--of the theological character of the Christianity of the relatives of Jesus and their circle in the first generation of the church. He has reason to connect the genealogy and the family of Jesus because of the statement by Justin Africanus found in Eusebius' EH 1.7.14 about Jesus relatives referred to as the desposynoi ("those who belong to the master"):
Most interesting to me at least was his discussion of both the early Jewish Christian mission in Galilee lead by Jesus' relatives from Nazareth/Kokhaba during the first generation of the Jesus movement and the Davidic messianism contained in the Lukan genealogy. Luke's family tree reveals that Jesus' Davidic line runs not through Solomon (as Matthew's does), but through the lesser known son of David, Nathan, who is linked then to Zerubbabel. The genealogy is not Luke's creation and reveals a pre-Lukan traditional genealogy that traced the line from David to Nathan to Zerubbabel.
The striking assertion by Bauckham is that Luke’s genealogy derived from Jesus' family. Thus, as Bauckham avers:
"We must now see the family of Jesus as Davidides, conscious, through family tradition, of the hopes of Davidic restoration which had been cherished in their line since Zerubabbel . . . the tradition may not have been important to Jesus himself, but it was there to be activated and developed when Jesus’ relatives became some of his most convinced and dedicated followers". 
 Bauckham 1990:1, emphasis added.
 Bauckham 1990:61, with my addition.
Bauckham, Richard. 1990. Jude and the Relatives of Jesus in the Early Church. Edinburgh: T & T Clark.