Free offline courses
I posted these links in a comment, but I thought it may interest others.
If you're looking to off-lines courses to assist your learning on duolingo, there is several free options availables.
Lernu of course: several of the lessons are downloadable as pdf and mp3 (lernu.net).
A multimedia computer program with online tutors at http://www.kurso.com.br
A course in ten lessons: http://esperanto-afriko.webs.com/kurso%20angle.pdf
The course: the esperanto teacher http://hg8lhs.ham.hu/languages/esperanto/The_Esperanto_Teacher.pdf
The huge textbook of Ivy Kellerman downloadable here : http://esperanto-edmonton.wikidot.com/kellerman-answerkey (there is also the answer keys for the 17 first chapters)
The last 2 ones are older, but I don't think the base of the language changed much, and anyway, done in parallel with duolingo and/or other sources, the potential changes should not be troublesome.