how to read the sourcecodes of u-boot

hello:
I am a newer about u-boot,and now I am trying to read the sourcecodes of u-boot. But it is difficult for me to understand it.So problems happens when i try to understand the program language,such as .text /.long /.global .now I can.t
find the proper books or articles to study it ,maybe ,someone of you could give me some hint about how to understand the program language or some books and some useful matiarials.

thanks ,,,and Merry christamas day!!