I've been offered a job in a majority Japanese office.
My Japanese is fairly conversant, I can get by in general conversation no problem. I've been recommended however to look into a book which teaches 'office Japanese', i.e. the language particular to working in an office that I will need. I'm told there are lots of complicated terms that one won't find in everyday Japanese but will find in an office.
Can anyone recommend any resources for this?
Books or websites.

I'm sorry, what's a majority office?.
What about googling ビジネス日本語　Amazon. ？

There are a lot of great suggestions posted already, if you search for forum topics with the word "business" or "business Japanese" in them. That's usually a more common way to say it than office Japanese.

A majority office? Majority means most of the people are something. So...majority Japanese= most people are Japanese.

Business Japanese is a different thing. Simply getting business Japanese book suggestions would be easy, as you say it has been covered lots and searching on amazon brings up mountains of them. Business Japanese books tend to be quite generalist about learning an upper intermediate level of Japanese with a bit of an emphasis on using it in office situations.
I don't need the generalist side of things, I don't need yet another textbook for learning Japanese, I need something specifically about the sort of Japanese used in an office.

Just learn it on the job, by listening to people, reading/writing emails etc. that's what I did.

May I suggest a simple google search in Japanese?
I came up with this fairly quickly→ http://www.digital-sense.co.jp/cc_new/sub/03_1.html

If you want the inner workings of keigo without a book seemingly made for infants (read: foreigners) you're going to want something made by and for native Japanese. There are a lot of references like this on the internet for young, Japanese business people.

Yeah, you could try searching something like ビジネスマナー. Or you could ask if your company will sponsor you to attend some sort of business manner course - I went to a one-day course held by Recruit which was fairly useful (mostly about manners and formalities like where to sit in a meeting, or how to exchange business cards etc., but it did also have some useful stuff about language).

I recently found this, too (a bunch of business email templates - useful not only for learning business phrases themselves, but also to understand the sorts of occasions on which you might be expected to send emails etc.):

http://business-mail.jp/example/%E8%B3%8...3%EF%BC%89

[Zgarbas]

I found 恥をかかない！すぐに身につく！敬語の正しい使い方 to be a really good book for this. It's mostly etiquette in the office&various turns of phrases; really educational, and it starts from really basic things to subtle changes, as well as things to look out for when opening doors/bringing coffee/using the phone/etc.
Pretty affordable, too; I paid about $20 for it. It's 100% in Japanese, though.