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I really like Graded German Reader, if you can find a copy anywhere. Great book, lots of annotated short stories in gradually increasing difficulty with grammar and vocabulary explanations. Helped me a lot when I was just starting out.
In addition, there's the much more recent Graded German Reader: Erste Stufe. Out of print but still in copyright, but there's a PDF floating around.
Studien und Plaudereien is very cute but obviously pretty outdated / old fashioned.
this book is a masterpiece, pity other graded readers aren't so well put together. This book genuinely can serve as a stand-alone bridge going from "I literally know 4 words in German" to "I'm ready to start reading children's literature with the help of a dictionary". To be fair, I guess you really would need to look up your own grammar (especially adjectives) but it really does do a pretty great job of setting you up with the basics. Just a shame that it's expensive and out of print. So es ist nun einmal.
Maybe it's time to move on from Duolingo? Where are you in the tree? If you're even half-way through, the rest (in my experience with the German tree at least) starts to get radically much less useful. I just finished my second Harry Potter book yesterday, and I just saw for the first time a particular word I'd learned in Duolingo... got me thinking, and I realized a whole bunch of other words I learned there I've literally never seen (granted, only two books, but holy shit there are a ton of other more important words that Duolingo could have gone over given my goals).
After all, frequency lists are all well and good, but if you're wanting to play Terraria with a friend, das Schwert und der Schatz is going to be much more important than das Geschäft und der Anwalt.
Anyway... maybe it's time to move on? If you like reading, try finding a copy of this book. I bought an actual copy, but I remember seeing that there was a free digital copy available at a library site I found, I can dig up the link if you want. I'd suggest you read it, and don't make any flashcards the whole time, just try and learn new words as you see them... it has definitions for new words as you go, so it's pretty painless, and gives you practice with learning new stuff as you run into it. This graded reader's amazing since it adds stuff in gradually, re-uses words with pretty good frequency (so you remember what you ran into) and by the end of it, I was able to jump into a 'real' book using a kindle to help with lookup. It's a lot more fun to read a book you're interested in than it is to spend time on a lesson, or going over flashcards. Those things both have their place, but you've likely already moved into the diminishing returns territory.
Audio books can be good too... I found a torrent of 88 graded reader audio books. Once you've got enough vocab under your belt that you know almost all the vocab you see there (should happen quicker than you think, since they're graded readers), those are great for listening practice.
Obviously somewhere in there too, probably worthwhile to start writing/speaking with someone. Either way, time to move on.