For my internship in Tanzania (researching erosion on Mt. Kilimanjaro, had a great time!) I learned some Swahili, and I used this book called "Simplified Swahili". I noticed that the course creators have this book too, and have probably used it as source material for this course. I recommend picking it up, it's an excellent guide to this language.
Only thing that I missed in the book is sufficient practice material (there are exercises, but I would have liked more of them), but Duolingo is an excellent supplement for that. Planning to work through the book and this course myself now, it would be lovely to return to Tanzania in the coming year and to be able to speak with the people better! :)
Btw thanks for making this course! It's great to see that it's in Beta now, I've been following this since it was announced!
I actually bought it in a bookstore in my university town the Netherlands (the town is called "Wageningen"... and interestingly enough "Wageni" means "visitors" or "strangers" in Swahili, I liked to joke about that). It's also available on Barnes and Noble, and Amazon. It's worth the money, I have met people in Tanzania who have completed this book (something I hope to do this year) and who say that it has been enormously helpful!
I have many different books that teach swahili and this one is my favorite.
Complete Swahili (comes with cd's and a book)was pretty good,I also have Spoken Word Swahili. My favorite dictionary is by Baba Mamlaka/swahili to English, but they have stopped printing it. I have other good dictionaries, but they are harder to carry with you. I have studied Swahili also using the pimsler method and rosetta stone(no longer available), but I don't feel they are as good even though I learned things from both of them. They do help you get used to hearing the language. "Before you know it" is good for building vocabulary and some sentences to be able to get around and you can develop your own lists. The talk now program is fun because it's like playing a game, but it's also more for developing vocabulary and simple sentences. Really what program and books I would recommend depends a lot on what your goals are. Some are better for learning the grammar in detail Like duolingo and others are better for learning enough Swahili to be able to get around if you go to Tanzania for a short trip.
Another resource that can be of help is a Standard English - Swahili Dictionary. It does help you grow the Swahili vocabularly with time especially for noun and verb focused students.