Apostrophe or not?

Thomas's book with the 's' is more usual - most usage guides say to only omit the 's' on certain famous historical or mythical names like Jesus, Mars, etc., and a couple of other exceptions.

It doesn't really matter as long as you are consistent.

It does matter, and Thomas's book is correct.

Dale

Yup, it's Thomas's book. The rule is that everyone gets an 's unless they're a biblical figure (Jesus', Moses').

The book I always consult for stuff like this (which my dad, a writer, gave me) is Hart's Rules (full title: Hart's Rules for compositors and readers at the Oxford University Press). That suggests always using s's except:
- words where the last syllable is pronounced 'iz' (like Moses', Bridges')
- classical names (Thermistocles', Mars')
- Jesus' (special case).
It also says that s' is acceptable but not as good as s's for names of three syllables or more without emphasis on the penultimate syllable: e.g. Nicholas' (but it still says Nicholas's is better).

So Thomas's is definitely write. And being a pedantic soul, I'd say of course it matters. Utherwize wats the poynt of hafing spellig or langwidge atol?

Both ways are correct, they're simply a preference in style; writing Thomas's is the way you're personally used to, but you wouldn't get a failing score in an English class for writing Thomas' - that's the way I was taught to write it, and that's the way I've always used it.

However, much as seeing Thomas's irks me personally - I, too, am a pedantic sort - I realize that it's a stylistic difference and that the apostrophe in any case is an over-stretcahed and ill-defined character. So I'll say it again: as long as you remain consistent in the way you use it, it really doesn't matter.

Unless you're writing for a publication with a predefined house style (and they'll all have one of those), in which case you'll of course have to write by their rules.

It is definitely stylistic and you can find recommendations for either style. (e.g. The AP Style Guide specifies only an apostrophe; the Chicago Manual of Style specifies "'s" with few exceptions)

Go with what you prefer. The biggest thing is simply to be consistent in how you handle it. (If you thinking about getting published, well, the publisher will have their own style guide and copy editors anyhow.)