Any detailed good books for astrophysics?

Im 15 year old, and have huge interest in astrophysics. I know the basics of the string theory, special relativity,etc and more about general relativity, dimensions timelines , blackholes , space time etc... I've read some books by michio kaku and stephen hawking, but they are mostly casual reading. I need a book more towards the serious side to tackle in my summer holidays. plz answe