Quite so, SurreyGuy...to some extent. Here's what the two 'bibles' of usage have to say on the matter...
a) Fowler's Modern English Usage suggests that, for personal names ending in 's', (quote) "In all monosyllables use 's for the possessive." That gives James's whatever. It goes on re longer names, "It is customary to omit the s when the last syllable of the name is pronounced iz as in Bridges, Moses, Jesus." That gives Jesus' whatever.
b) Gowers' Complete Plain Words says re monosyllables that the addition of the 's is (quote) "certainly the commonest practice but with longer names many people let an apostrophe do the job alone."
In other words, the two are in virtually complete agreement. That is, it is not actually WRONG to write "James' book", just uncommon. Similarly, some people write "Dickens' novels..." whilst others write, "Dickens's novels..."
Ye pays yer money and ye takes yer choice, as they say! Personally, with a single-syllable name such as James, I would never dream of writing "James' book" and nor would MOST other English speakers.