RHCSA/RHCE® Red Hat® Linux® Certification Study Guide, Seventh Edition (Exams EX200 & EX300) by Michael Jang and Alessandro Orsaria
We have used Michael Jang's books for previous versions of our Linux courses. It is a very affordable text. This is a great book that walks you through the starting stages of learning Linux through getting certified RHCE. It may be advanced for some readers, but I recommend you consider it for your professional library.
A Practical Guide to Fedora and Red Hat® Enterprise Linux®, Seventh Edition by Mark G. Sobell
This is the textbook we used for IST190 in the 2016 spring semester. It is very thorough. It is an excellent reference book to use while learning Linux.
Linux Pocket Guide by Daniel J. Barrett
This book only costs about $9.95 new. I just bought a used version for $3.50. It provides an indexed list and explanation of most of the Linux commands we will be using in IST190. Many IST190 students have bought this little book in years past.
Red Hat Linux Networking and System Administration by Terry Collins and Kurt Wall
This book is a little old but it includes a ton of valid information about Red Hat Enterprise Linux. I used this book when I was learning RHEL.
Linux Command Line and Shell Scripting by Richard Blum
I like the "Linux Bible" series of books, but I do not have the current version for Red Hat. New "bibles" come our every year so it is kind of hard to keep up.
If you find a Linux resource book you like, please share it with our class.