Best books for learning/studying German.
As much as I love Duolingo, I feel it doesn't explain Grammar or sentence structure to a full extent. In recent times, I have been studying with books to understand grammar to a better extent. Here's my top 5, to complement your learning.
Living German (Hodder Education) http://www.amazon.co.uk/Living-German-Grammar-based-Course-LL/dp/0340990759
German grammar, made easy (Lisa Kahlen) http://www.amazon.co.uk/German-Grammar-Made-Easy-Kahlen/dp/0340904968
The everything learning German book (Ed Swick) http://www.amazon.co.uk/The-Everything-Learning-German-Book/dp/159869989X
The key to German grammar, for key stages 3 & 4. http://www.amazon.co.uk/Key-German-Grammar-Stages/dp/0748719237/ref=sr_1_1?s=books=UTF8=1398796648=1-1=The+key+to+German+GRAMMAR
Ich hoffe, sie helfen!
Great post. :) There is also another book that might be helpful:
English Grammar for Students of German: The Study Guide for Those Learning German
The title is a bit misleading, perhaps: it's a book about German (!) grammar, but German grammar is explained by comparing it with English grammar and by demystifying terms such as subjunctive, indirect object or subordinate clause. :)
I have been leaning German for the last few months. I've tried an intensive Deutsch course in an actual Language school, besides of course Duolinguo, Rosetta Stone and other online resources. It has been only until a friend recommended me using a textbook called "Jens and Jakob" text book that I felt my German was really taking off.
The course is written parallel to a funny story/plot and it has very illustrative resources and subsections. From my experience this combined with Duolinguo has been the best available source for rapidly grasping Deutsch at my very own convenience.
Here is the link: https://www.skapago.eu/jensjakob/
Good luck to all! / Viele Glück!!
I'd also recommend '501 German Verbs' by Henry Strutz:
I refer to this pretty much every time I need to write or email in German.
Just get Hammer's German Grammar and Usage. You can get Practising German Grammar along with it, the lessons correspond to HGGU.
Ich benutze dieses: http://www.amazon.com/Practice-Grammar-German-Edition/dp/3190272557 und ich finde,dass es sehr nützlich auch ist.