EMBOK stands for the Event Management Book of Knowledge (Event Management Bok).
The concept, in theory, is similar to that of the PMBOK, but on the Event (and not the project) level. I have researched EMBOK and it doesn't seem to be something official or issued by any official organization (such as the PMI). It seems to be that this is the effort of one individual to elevate events to the rank of projects.
In fact, my research about the subject only lead me to EMBOK's website - there doesn't seem to be an actual EMBOK book!