I am currently involved with an installation project of SAP in several countries and I have had the dubious pleasure of getting exposed to the ABAP code. If you are involved with an SAP project you know the software will contain some bugs and you will also want some customization. Unfortunately, most of the programmers out there do not know your business and at times it is frustrating bridging the gap to meet the customization needs.
So the more of the programming language that you know the easier the communication will be. For example, the programmer will give you a program and if it does not meet your needs it is very helpful in being able to read through the program logic and understand what the program is doing. This book does an excellent job in explaining the commands, the syntax, giving practical examples, and highly stresses the practical rules for optimizing the program.
If you have the ability to write your own programs and play around in a development system this book is the one with which to start. It will give you the basics and beyond and is very easy to read and follow, especially with the examples.
Again, I highly recommend this book from a practical reference and also as a text book to learn how to program in SAP.