Comparing two columns/sheets

Hi,
Would be grateful for any advice - I've trawled the web for a solution, and am sure this can be done but need some pointed advice.

I have two sheets, both with client data. Eg Columns A-D as follows:

A: client code
B: first name
C: last name
D: email

'Book 1' is say, a master list of clients, with the 'first name' contact being the primary contact. 'Book 2' is a list of clients who are registered for an event. There are multiple client codes in this sheet. For example, there could be multiple rows with the same 'client code' but different 'first name' etc. Client codes are numeric and range from 1-10000
My objective is to have a list of clients from the master who are not on 'Book 2'. Should be simple (but so am I!).

This is fairly basic but should do the trick.