Thanks, Steve, for your detailed comment!

I find it interesting that you find Duolingo better than other tools to practice grammar as, on the contrary, I find it quite ineffective for this. I started Japanese with tons of exposure combined with textbooks and grammar study, and it worked wonders for me.

To get a "feel" for a new language, I believe resources like Assimil are much more efficient than Duolingo (and other such apps, for that matter) as they explain patterns in few words and re-expose them to you regularly enough to assimilate them.

I can't talk about Finnish though, as I haven't studied it at all, but I'll keep your recommended book for the day I turn to it!