"Pilots have been saying for years that the majority of whats taught on the ATPL syllabus is not actually required knowledge on the job, but is it?"
I kind of get tired of saying this, and I'm sure other schools do, too, but it is. Probably about 5-10% of the stuff is probably never going to be used, but it is still useful background knowledge. SOME knowledge is not required for SOME pilot jobs, but it is a profession that demands flexibility. All the JAA want to do is ensure that you know it before you start, for which I don't have a problem (but don't get me started on the questions - and Rithalic - you are quite right about the exam structure). The syllabus has been "updated", which essentially means moving subjects between the learning objectives.
I can assure you that, after you have about 8000 hours and knocked around the world a bit, it will all have been useful somewhere - I still use ch long mentally bouncing around N Alberta - and the whizzwheel after 30-odd years!
Speaking of bouncing around the world, it is likely that you will get more than one licence - absorbing the core knowledge now means that you are in and out of the exam room as quickly as possible and you're not continually learning yet another set of dodgy questions.
In answer to the original question, I continually read books, some of them over and over. I leave you with this quote from Nick Lappos (a very senior engineer/flight instructor):
“Pure book knowledge should be impeccable - every second of doubt about "what do I do now?" is worth 30% of workload. Mostly because the self-doubt and second-guessing are real time and mental capacity wasters. The more you know flat cold, the easier it is to fly under the gauges”