Plural name apostrophe position

At my English lesson the native English speaker couldn’t tell what is correct and promised to search it for us!

He told us that if James is one person then we should write

James' Book

but if we had many James he didn't know to tell us what to write...

Any suggestions?

This is something that's not 100% settled in English orthography, so it tends to be set down in usage guides. For example, here's what the Chicago Manual of Style (14th Ed) has to say:

"6.24 The general rule for the possessive of nouns covers most proper nouns, including most names ending in sibilants."

Examples they give include Kansas's, Ross's land, and Jones's reputation. Exceptions include Jesus' and Moses'.

By this, it would seem reasonable that possessive of the plural would be James', as with common nouns.

Note that this is not the entire story, but might (?) be the relevant bits.

First of all, see my answer here regarding the correct way to make a possessive of a name ending in S. (Short version, both James’ and James’s can be considered correct).

For possessive plurals of names ending in S, you first have to form the plural. Like any noun ending in S, the plural adds -ES, so one James, two Jameses. For possessive, just add an apostrophe: Jameses’. This formation common for last names (“keeping up with the Joneses’ spending habits”) but can also be used for first names. For example, the dog belonging to James Smith and James Jones would be the Jameses’ dog.