Here are some books regarding Linux and Ubuntu that I find pretty good. These books are available to purchase at Amazon.com via the links below. A short overview is provided for each of them.
Ubuntu Unleashed 2014 Edition: Covering 13.10 and 14.04 (9th Edition)
The Official Ubuntu Server Book (3rd Edition)
Linux System Programming: Talking Directly to the Kernel and C Library
This is a power book for programmers and developers. It covers the more intrinsic concepts about how to use system calls and communicate directly to the kernel in applications written in C. You can buy it from here or click the image below.
The Linux Command Line: A Complete Introduction