If you only get one book, get this one. It is more about 'why' than about 'how' and shows what tradeofs are involved in one or another design, what problems you can run into with different solutions. Also, there is a new book by Sandi coming out: http://www.sandimetz.com/99bottles/
Her talks are highly recommended too.
Sandi Metz is great.
This talk is such an amazing example of clear thinking:
I actually watched that talk twice. The technique of introducing a seam set off light bulbs in my head that didn't go off the first time I watched it.
Seconded. As a self taught generalist ruby/python/java guy, I read POODR two years ago and it stands out as a great investment of time. Lots of "a-ha, that's what I should be doing" moments. Has anyone checked out her new 99 Bottles book?
I was about to chime in with the same message. An important part of this book is that the language used--not just the code language, but the writing language as well--is very easy to follow and understand. This is not a book you'll regret reading.