Hi Layla ...
the best books which I always have with me on my InterRail travels are the books of the Lonely Planet series ...
because this are special books for backpack-travelers updated every year ...!
The books called interrail handbook or so on, only have some more special inforamtion about interrail in it, which you can also find here on the website.
so no need to buy them - and they are normaly not filled up with so much information like the Lonely Planet books.
Have a look at the website: preparation - literature