A book launch event [to be held in Berlin.]
If the launching of a book is one of many similar initiatives that was held in Berlin then the indefinite article "a" is appropriate. If instead the OP is mentioning a book that the reader is familiar with, then I would use the definite article: "The book launch".
I am not convinced that including event is always redundant as suggested by one user's answer. In my opinion, an event hints at a party, a stuffy presentation, or the opportunity to meet the book's author in person. A launch, on the other hand, could be limited to an official announcement in newspapers, radio and on social media. It could be rather low-key and on a tight budget.
- Illness prevented her attending the launch party for her latest novel.
Remove "party" and the meaning changes.
- The event got off to a shaky start with the stage lights failing in the first few minutes.
Replace event with launch and the reader would know what the event was and guess the venue was perhaps a theatre or television studio.
However, a book launch event sounds awkward to my ears. The following are suggestions but by omitting "event", the launch sounds less significant.
- The book launch planned to take place in Berlin last summer was cancelled
- The event planned for the book launch in Berlin was cancelled last summer.