Programmable Logic Controllers is an excellent reference book on theory and operation of PLCs used in the control of production systems. The text offers practical introduction to the structure and use of PLCs in industry, covering both programming techniques and systems-level design issues.
It's up-to-date on the latest technology, it's comprehensive, and most of all, it's easy to read and understand. The book can serve as the textbook for senior year graduate course on the subject of automated process control. It is also worthwhile purchase for anyone interested in learning more about this very important field in control engineering and instrumentation.
If you need a good reference book on PLCs, it will be good to start with this.
According with the above mentioned, I highly recommend this book.