Learning electrics for the exams is not the same as learning electrics! Unless the book/CD directly targets the ATPL subjects, they will be a waste of money. You want to be a pilot, not an electrical engineer.
Any good pilot study book (like those recommended) will introduce you slowly to the topic and teach you what you need to know to pass the exams.
I was a radio engineer before flying and confidently (red cockily) thought I didn't much study for the radio aids exam. How wrong I was. The exams asked questions that were either not straightforward, discussed old technology (Decca etc) or were just plain bewildering. A resit was needed and after a brush up at PPSC, they had me ready for the exam, 100% at second sitting!
Save your money and spend it only on the materials you need to pass that exam.