Q: The book seems to suggest that Judas the Galilean was the basis for the stories of Christ. Is this what you're saying?
A: Not entirely. That's what it says in the book – but that's because the book is a snapshot of a very short period of time in the mid-60s AD and the bulk of the myth creation happened later, after the fall of Jerusalem. See 'Caesar's Messiah' by Jospeh Atwill http://www.caesarsmessiah.com/main.html for a more detailed explanation of why Titus Vespasian wanted the force the Jews to agree that he was the Messiah. I don't agree with everything Atwill wrote, but most of it makes eminent sense.
As far as Judas being Christ, I think he was part of the model for the myths that arose later, but Judas was a warrior, who led his Sicarioi – also known by Jospehus as 'The Fourth Philosophy' - in a highly effective insurgency against Rome from at least 6 AD when he raided the armoury at Sepphoris and armed his followers (and the Romans then crucified every man in the town and enslaved the women and children) until his death.
We don't know much about his death – if Jospehus wrote of it, the details have been erased - but if he was captured, he will have been crucified, thus giving Paul his starting point.
Later, however, there was a need to take this minor sect and expand it. At that point, some kind of life narrative was necessary and was created over time – the last three years first, woven round the half-remembered stories of John the Baptist to give it verisimilitude, and then the birth stories a decade or two later, with their even more poorly remembered concatenation of dates: Herod killed his first born sons in 6BC. The Census of Cyrenius took place in 6AD, after Herod's death.
These things are clearly fiction, poorly sourced, but somewhere along the line, someone spoke or wrote the words of the sermon on the mount. I doubt very much if that was Judas, but it might well have been James, his brother, the man who was a Nazirite, given from birth to the god; a vegetarian pacifist who held peace in Judaea against really quite overwhelming odds from Judas' death sometime in Pilates' reign (either 17 or 26 Ad – 36 AD) to his own death in 62AD.
I think, therefore, that while Paul based his creed on Judas, the Christ of the gospels is based largely on his brother James, with odd bits added from, for instance, Menahem, Judas' grandson who, in 66AD, rode into Jerusalem on an ass, proclaiming himself the king of the Jews (he died soon thereafter, tortured to death by the son of the High Priest). The later writers, though, did have to do everything possible to distance their new pro-Roman religion from the savagely anti-Roman bias of the Sicari zealots. They did this by using the name of 'Judas of the Sicarioi' in their narrative, altered just enough to be less recognisable: Judas Iscariot. In one of the most successful, blatant spin operations of all time, they made Judas his own worst enemy – but they had to wait until the men who could have said, 'this was not so' were all dead, which meant none of this happened until after the siege of Jerusalem. It's possible the Ebionites, who call themselves 'The Poor' were the natural inheritors of James' Jerusalem Assembly: they consider James to be their leader and Paul to be an Apostate – but they're suffered 2 millennia of persecution, their original creed, too, is lost. (see 'Judas the Galilean' by Daniel T Unterbrink for a very cogent analysis of Jesus as a title for Judas in the way that 'the Boudica' was a title for the woman who led her armies against Rome. Clearly, from what I've said above, I don't think Judas was the whole of what became the myth as Unterbrink does, but it's a good starting point).