This means upgrades are very costly. Customization should be avoided for easier upgrades in the SAP software. A work process connects with the database and has an ABAP language interpreter and processor. There are lots of jobs for ABAP programmers. There are many web sites of headhunters and even people's resumes.
Publications are available on the web and they are pretty technical.
There are a lot of sites for programmers to share information and get help. Of course they all want you to sign up with them and sometimes you can't get any information unless you do. Sometimes these sites will have a dictionary or encyclopedia. It is by Andovernews. They encourage exchange of articles written by members.