They say that 34 hours on Duolingo is equal to a university semester. It seems as if they do teach proper grammar. My husband is fluent and he said that what we were learning sounded spot on. I’m not sure if you could go straight to reading books after you finish it or not.
My husband used to work for an Italian company and he was able to learn enough Italian on Duolingo that he could understand and sort of converse with his Italian boss and co-workers.
I think it helps if you can practice it in real life as you go. Seems like we retain it better if we try to converse with each other and with my husband in Spanish every day. We also watch Spanish shows with the subtitles turned on and that helps to fill in the blanks, too.