Mój kolega Alfie ma pytanie

which spelling is british and which is american: realise, realize, organise, organize, organization, emphasise?

-se / - sation - British
-ze /-zation - American

In many british books I find realize spelled with 'ze'. Why?

because there is a difference only in terms of the suffixes, and not the style in which articles are written

So what you are saying is that British authors copy Americans and write in their style?

not exactly; what I'm saying is that nowadays the particular suffix used in an article doesn't depend on whether the author is British or American, or the style in which they are presented, since both American and British English are actually the same language; so the distinction between the two different versions have in some way been fading away in the recent years

Is this your personal opinion, or are you sure that this is formally correct

what I've just written above doesn't normally refer to the suffixes '-re' and '-er', which are still distinctively used in British and American English, respectively; the same goes for '-our' and '-or', like in 'neighbour/neighbor', and some other differences

>Is this your personal opinion, or are you sure that this is formally correct

formally correct which I share my personal opinion with :)

Thanks. That is the first time that someone could give me a proper explanation.

Gucio..
zwroc uwage tez na to, ze ksiazki ktore sa wydawane dla czytelnikow US - maja pozmieniane slowa na AmE.

But this was a British book bought in Britain.

Yes, but to be absolutely sure (of what the spelling etc should be) you would have to know the nationality of the writer, maybe he was American, maybe he studied in America etc.
What makes you think that only British people live in Britain.