There are at least two different sets of issues you should be looking for:
- Knowledge of ABAP as a programming language
- Knowledge of the "Business Domain" that your writing your software for and its implementation in SAP - tables, forms, programs, reports etc, (and each of the modules such as FI HR etc. is more than a normal person can usually be proficient in)
(1) gives you general knowledge on how to write a program, read and update the database, and maybe write a GUI. But the programs that you write will almost always be in the context of (2), so you will need to know that as well.
If you want to get started, it is best to have some general knowledge of the ABAP language, the business domain can't really be learned from a book. Actual project work is much more helpful.
