I wouldn't recommend buying any of the additional reading books - if you really want them you can go to the library, but my experience was that the online databases are much better. Best piece of advice I would give about reading extra pieces is to find a couple of journals/articles, skim over the introduction & conclusion and get the gist of what the author's argument is, and then pick a quote or two that shows that and learn that. Referring to journals/academics in exams is going to really give your marks a boost.
In terms of reading, I did nowhere near all of the essential reading, never mind all of the additional reading. You quickly learn how to read in a smart way and not just read endless amounts of rubbish on the pretence that reading more has to be a good thing.*