There is no doubt that Google totally changed the search engine optimisation (SEO) world when it launched the Penguin update in April 2012. Plenty of people had been affected by the earlier Google Panda algorithm update but that was nothing compared to what happened with Penguin.
The book, SEO 2013 & Beyond by Dr Andy Williams is the first thing I've read (and I've seen plenty of blog posts about Penguin and how to recover from it) that makes sense of what Google did with this fundamental change.
All of a sudden, all the things that you used to do to get ranked high up in the search engine no longer had the positive effect you wanted. In fact, if you overdid your SEO (onpage or offpage with link-building), you found yourself penalised.
SEO 2013 & Beyond by Dr Andy Williams
Business Book Review - 4 Stars
If you've suffered from the Penguin update or you want to rank high in the search engines by using SEO in future, I believe you need to read this book.