Danny - I think your site is a really good one. It looks great, loads fast, it's simple to use. I wouldn't mess with the template or look - it works well as it is.
The only thing I can think of is that perhaps you need a bit more text on the homepage.
As to why G dropped you - I think it's to do with Amazon links on all your review pages. For instance, you have a review linked from your home page for a PD James book called "Talking About Detective Fiction".
If you google "talking about detective fiction" (without quotes), amazon.com comes up top.
I think the new algo assumes that if amazon is in the top ten, then an affiliate also touting amazon is superfluous to requirements, regardless of whether they have value that the amazon pages does not have.
Are you able to subsitute those amazon affiliate links with another bookseller? Says Barnes & Noble (I think they are on the google affiliate network). It's worth a try - you lose nothing, as your traffic has tanked - some sales through barnes and noble and the other publishers (I think Random House have an affiliate program through shareasale, most publishers have one) is better than no sales with amazon because G is penalising amazon sites.
P.S. if it doesn't work, you can always put the amazon links back. Switching affiliates is not a drastic change though it may be time consuming.