SAP Associate Level Exam preparation

First, let me make your core basics clear.

When you are referring to SAP ECC, you are referring to SAP ERP Central Component which is more or less equivalent to the prior SAP R3 System.  ABAP does not play any much part over here.

But ABAP resides in the kernel of SAP which is referred to SAP BASIS.

Speaking about your pointers for reference

• Horst Keller - ABAP objects (For ABAP)
• Help.SAP.com  - (overall SAP)
• SAP ECC 6.0 Black book

 Then there are blogs, Q&A, and other online help always available. You can start from anywhere and when you move along, you will discover online assets as well.