The problem with just textbooks is you don't get to hear or speak the language at all. There's also very little structure so you're expecting a novice to know how to effectively test themselves.

The beginner textbooks I've used have accompanying CDs, and also audio available on their websites. The best beginner listening practice is to focus on minimal pairs so that you can start to hear the difference between different sounds that originally sounded the same. Some textbooks include this (I remember having to combine initials and finals in Chinese with my teacher)

For speaking, Duolingo speech recognition isn't very good and is not going to provide specific feedback for improving your pronunciation. I'm not even a fan of practicing pronunciation virtually with a live tutor. I have found in person pronunciation practice to be very valuable compared to doing the same thing but online. A textbook doesn't solve this and neither does Duolingo.

They are also boring to the extreme and don't bite size it for you. They are also unpactical format- fiddling with cd requires you to be at home and have cd player available when you want to learn. So basically you have to be near laptop.

I don't know anyone who did stick with textbook approach long enough to learn something.

I also used those things! I used Anki and Quizlet (which isn't open source but you can import your own word lists so it doesn't feel too "sticky"). Duolingo was still key, though - it felt less like work, because it was nicely gamified. (I give all this praise with a twinge of dread now, because I'm sure that with the IPO, Duolingo will be twisted in the way that all publicly-traded companies are).