If you don't own a copy of the C library book, there's c standard library site which is not detailed as the book but it's online. Not only declarations but also definition are provided.
C standard library [clc-wiki.net]
It has section with solutions for K&R2 book.
The Open Group Base Specifications [pubs.opengroup.org]
The official definition of a UNIX system. Functions are indexed and quickly retrieved. I use it from time to time as a reference.