when you say office management is it more running the office or managing other people, or a mixture of both?

When I did my secretarial training 10 years ago a lot of the secretarial course textbooks for all round training (I did the LCCI Private Secretary's Certificate course and studied for the diploma exams at the same time, squeaked through somehow with a few practice papers) included a lot of information which would be useful, I'd suggest looking for a handbook aimed at fairly senior secretaries.

If you're moving into managing other people, HR, health and safety, maybe a general management/intro to management course would help, I know it's something some of my PA colleagues have done a bit of.

Luci