It is noted that you should only carry the essentials since walking a month with a backpacker is not easy.Books that are interrailers usually buy are the Thomas Cook Timetable Eurpean, and the Lets Go Hosteling International that indicates the youth hostels belonging to hosteling international. It is not essential since those books are a bit expensive, the train stations also have tourism offices that can provide information on accommodations, city maps and more! On the internet you will find all sorts of information ranging from travel sites, maps of countries, maps of railways lines, information about youth hostels, camping sites, etc... Whenever I came to a new destination the first thing I did was to go to the tourist office in the train station or in the city to gather information. For the more planned ones eventually when you arrive to a destination you can start searching information of a future destination, the days and hours of departure, times, if there is a reserve requirement for holding an Interrail ticket (therefore paying reservation), etc.
You should also get the European Wealth Card since it is a form of medical care in the European Union, you can request it normally in the Social Security centre (or at a regional delegation), the Youth Card and / or International Student Card (provides discounts at many museums , shops, etc.).
DO NOT FORGET THE CAMERA! Even if it is heavy and takes up space! Photos are memories that last forever and with photographs you will remind funny episodes. Do not save on the photos because when the Interrail ends those photos will be worth gold!
Anyone that wants to make an Interrail will have my 100% of support, perhaps the most difficult part is the first step because there is always that fear of being by yourself, but then everything becomes easier and is an experience that stays in your memory for life.