I am from Germany as well, so I can relate to you.
Starting with you monthly bills, I cannot see much room to improve, but there still is some room:
1) Internet for 35
I pay currently about 15 for my Telekom 16.000 DSL (the best I can get... High speed wasteland...) and lately I saw an even cheaper offer for a Vodafone 100 Mbit line on MyDealz: https://www.mydealz.de/deals/vodafone-internet-phone-cable-100-oder-unitymedia-2play-jump-120-ab-rechn-10-eur-monat-gratis-sonos-play1-971825
Since you have time to shop around, I would recommend watching this page for offers.
2) Cellphone for 45
I don't know the details of you contract, but you can get 3 GB with unlimited texts and minutes, without shopping around, for 10 per month. Based on this, your contract forces you to buy a new smartphone every two years for 24*35=840. That's very expansive and not a bargain.
You could buy last years model (the S7, a phenomenal device, is available for around 379) or only upgrade when and if it's really necessary and save these 35 extra Euro every month.
Combining the cell service and the smartphone in one contract makes even less sense, since you can easily afford to buy the S8 if you want to. And I am pretty sure that you won't have to pay 840 for it.
3) Your car
Switch to a smaller, used, gasoline car. This will reduce taxes and insurance.
Now regarding your last question:
We have quite a few good financial bloggers in Germany:
to only name three.
If you are looking for books, the blogs often have a book section. Many classics are also available in German, like Rich Dad, Poor Dad. If you are looking for something more practical, check out "Souverδn investieren mit Index-Fonds" from Gerd Kommer (https://www.amazon.de/Souver%C3%A4n-investieren-Indexfonds-ETFs-Privatanleger/dp/3593395428
You can invest in ETFs, all you need is a broker. The magic word is Sparplan, with a saving plan you can buy the ETFs monthly with little or no fees.
Robo Adviser like Bettermint exist in Germany as well. I know two, but never looked into the details:
Bonus tip: check out the 5 Ideen YouTube channel. They offer a great overview over several financial books.
I hope I could help you,