Orson @2320 - apostrophes
I couldn't find the Jesus and Harris examples, but I read elsewhere about a good drill for the possessive apostrophe. It avoids worrying about singulars, plurals and what does or doesn't end in s.
1) Who is the 'owner'/who are the 'owners'?
2) Write down the owner(s).
3) Add an apostrophe.
4) Add an s if you need to (to make it sound right).
I agree therefore with the BBC's Harris's book. For James there are probably two correct options: James' book and James's book, depending on your own speech pattern.
With Jesus I think you can get away with Jesus's preaching and Jesus' preaching, depending on context, but Moses's mother sounds ridiculous so it has to be Moses' mother. Both would probably benefit by avoiding the construction altogether (the preaching of Jesus, the mother of Moses).
However, as demonstrated by the Men's Singles, children's clothes and all the girls' exams, the above drill should work every time.