In May 2019 I need to take a train from Prague to Berlin. Any need to book in advance? What site is recommended?
The tourist season is starting to pick up in May. Personally, I'd feel more comfortable booking a train unless you're flexible on your arrival date for Berlin. If you need a back-up, there are buses that run from Prague to Berlin that were pretty good, at least as of 10 years ago.
I'll let someone else recommend a site, because it's been so long since I've booked a train out of Prague.
I think you should find everything you need regarding trains running between Prague and Berlin (both directions) here, including booking sites and guidance on booking in advance: