Books for intermediate/advance students

Most of the books about Buddhism I can find were made for Beginners, most are very good and touch the key points, easy to find and to buy. The problem is that after a while I started to look for more advanced books, made for people that have already studied the key principles of the Dhamma and wanna take one step further, this is very hard to find! Anyone can indicate a book about Buddhism (suttas, Abidhamma, techniques etc.) More suitable for advance students?