which online course - duolingo , rosetta stone, babbel - and a grammar book

The Economist has just published an excellent review of online language courses.

http://www.economist.com/blogs/johnson/2013/06/language-learning-software

It also has  a link to a review of the new rosetta stone.
I consider rosetta too expensive - have heard it can be found on tpb, too.
Maybe the new version is good enough, the elder ones were rather too simple
to pay for it.
Duolingo I never tried, but as it is free, it is worth a look.
Babbel I recommended years ago - I still consider the free COMMUNITY the
best feature about it: Ask others to correct your texts/homework/letters - ask
what ever you did not understand about grammar etc. - there are lots of kind people out there
eager to play teacher, and till you reached advanced level yourself, even an
un-professional help is helpful. (I have also seen many "real" teachers not competent
in explaining grammar. Believe me: They do not learn it in the German university! And
abroad they learn a lot of "grammar" - but often from old books, often with mistakes, nearly never
with "Grundgrammatik Deutsch"

von Jürgen Kars und Ulrich Häussermann - still the best ever to understand German grammar. (You need level B1 to understand it, it won´t help much if you are really advanced. But I would not trust a teacher who has not read it to be able to teach grammar. ... )