Due to the central location of the conference at the library in the city centre of Rotterdam, there are a number of hotels in the direct vicinity. Here are a few suggestions:
Stayokay (Youth Hostel) Rotterdam: literally a stone's throw away from the library. Hosted in one of the famous 'cube' houses. Prices range from € 28 per night for a bed in shared 8-person bedroom to € 75 for a 2-person bedroom with private bathroom. Breakfast included; it is a populair place with tourists so book early!
CitizenM Rotterdam, across the street from the library / market square. Built in 2014, so spanking new. CitizenM has only one type of room, the price depends on the type of booking (fixed, non-refundable or flexible) and range from ca. € 90 to € 127 per room per night.
IBIS Rotterdam is located less than 5 minutes away from the library at the Wijnhaven. With all the regular comforts from Ibis, rooms are available starting at € 77 per night. This hotel also has rooms for people with a disability.
If you want something different, the floating H2OTEL may be just for you. Also located at the Wijnhaven it is less than 5 minutes walk to the conference. Prices start at € 80 per room per night.
Another cheap hotel is Hotel Campanile, starting at € 47 per night. It is located outside of the city center but the advantage is that it is close to Rotterdam Alexander, from where the metro runs straight to Rotterdam Blaak (next to the library).
Note: all prices are indicative and do not include breakfast, unless stated otherwise.