Looking for university/graduate-level texts on RNA
As suggested by others, you can start with some basics on molecular biology. Genes - Benjamin Lewin and  Molecular Biology of the Gene - James Watson et al. are good books for the basics. 

I haven't seen any book dedicated to RNA-biology and most of my understanding has come from reviews. 

You can start by studying RNA secondary (and higher order) structures which is essential for understanding how riboswitches function. I would suggest that you first begin with the wikipedia article on this (it is not that great). The above-mentioned books would also cover the basics. You may then have a look at this review which is about thermodynamic and kinetic aspects of RNA-hairpins:

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Structures, Kinetics, Thermodynamics, and Biological Functions of RNA Hairpins

For Riboswitches you can refer to these reviews, which are quite comprehensive:


Ribozymes, riboswitches and beyond: regulation of gene expression without proteins
Riboswitches: Emerging Themes in RNA Structure and function


This is another comprehensive review on ribozymes:

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Ribozyme Structures and Mechanisms



PS: Some of these articles may not be freely available. You can mail the corresponding author (email address would be mentioned in the article) requesting for a reprint.