Babylon Berlin is based on the novel "Der nasse Fisch" by Volker Kutscher, but the production changed the story and the characters quite a bit, so much so that I wonder if further seasons can still be based on Kutscher's books.
Since I had read the books not too long before watching the series, I got confused because of those differences, mixing up the images in my head and on screen, so make sure to leave some time between reading and watching.
I can really recommend the books, they are well reasearched, they are historically accurate and give many nice details about the Berlin of that period.
I'm not expert enough to say if the changes and additions made in the series are still as accurate, but the production team consulted a historian while producing it.
Babylon Berlin is the most expensive German production so far and you can see it. Where cheaper productions have to go with 3-4 takes to get it done, here each scene was shot until the director was finally satisfied (I was an extra in this series, so I know).
Productions of such magnitude were not new to the team, many "Hollywood" movies and US series were shot entirely or in part in Berlin and Studio Babelsberg, so they had an experienced crew, headed by Tom Tykwer.
So is it any good? I'm not quite neutral, since I was there on set, and I dislike it when a good book gets altered too much in filming, but the images alone are worth it, you can see the extra money spent, so give it a try!
Edit: Review of season 3 (which is based on book #2 by Kutscher).
Again I was there as an extra and got a look behind the scenes. I couldn't find any numbers, but I had the impression that this season was done on a smaller budget than the first two, e.g. we didn't take quite as many shots on the same scene and seemed to be more in a hurry. Still, the sets, the costumes and some other things that cost money were already there and build upon. After watching the series on TV, I didn't really notice much of a difference when it comes to production value.
In this season, Gereon and Charlotte are undergoing some changes, thus becoming more like the characters from the books, to me these changes make them more amiable. Also, the plot follows the one from the book more closely than the first 2 seasons did, but there is still an entirely new sub plot.
Basically, if you liked the first 2 seasons, you will also like the third. It's a bit less glamorous, instead we get more detective story parts and you won't get as many history lessons, but you will get to see how movies were made 90 years ago. One of the great things is that the scenes that play at the Babelsberg Film Studios were actually shot there, they have survived a war, the GDR and the changes that came with the fall of the Berlin Wall.
