Comprehensive Linux API manual by subsystem Required

[Sasha Mckinsey]

Hello,

I know this is not a question which belongs in the Newbies list but there is a strong chance that someone out here might know about such a resource. 

I am developing a user space program which interacts with "file system" and "cryptography" so I looking for a comprehensive on the lines of "MSDN in Windows" which talks about the API by subsystem. Kindly provide the URL if you have such a manual/ resource in mind which is available on the net.

Thanks!