I'm a fan of fat events, and let the receiver decide if they want to trust the event or not, or go ahead and make a call to the service to get the data.

for example:

if one receiver wants to know if you have read a book, then there is no reason to make a call to the service.

but if a service wants to know the last book you read, and doesn't trust the events to be in order, then it would make sense to just call the service.