Play what you think will suit you best, and what books to read will depend on what you decide to play as they all have a different strategy.
There are some applicable accross the board however, like those to help develop the mental skill necessary to deal with the ups and downs of the game or the math side of things etc.
What if any form of the game have you played up to now? Was there a particiular form you enjoyed playing more or felt you played better than others? Is time constraint an issue? Do you often have to stop on a whim for whatever reason?....so many questions to ask that would determine what might fit best but its going to come down to the format you enjoy and feel you could be most profitable in.
To study and learn you need to get some enjoyment out from the format as you will just not be motivated if for example you hated playing sng's but decided to play those.
100eur br is certainly a good br to get started on and maintain a decent brm playing <=1eur games.
Where to start... well 100EU is a nice BR I also love to play 6max my advice is play low tournements from 1 to 7 EU (use up to 10% of the BR) but nice and slow game, pick ur cards they will be ur tools think about them. Be aware of aggresive players if someone re-raise think about that also even if u floped the top pair.. maybe the guy haves 2pair or over pair If u realy insist of reading a book check this "Phil Gordon’s Little Green Book: Lessons and Teachings in No Limit Texas Hold’em" and think "Poker is easy to lear but takes a sh*tload of time to master it" GL 'n' HF