I do not need accommodation, can I also only book the course?

Yes, for almost all courses it is possible to book the course without accommodation. You can select if you want housing in the application process.

For tracks that take place in Italy or Germany, housing is mandatory.

Please be sure to have a place to stay during the Summer School period; finding affordable accommodation in Utrecht and surroundings is not easy. Owner-occupied houses are expensive, rents on the private market are rather high and you have to be on a waiting list for a few years before qualifying for social housing. Of course there are always hostel/hotel or Airbnb options available.