Hello! I am an IB student working on my Extended Essay in which I am investigating the impact of the First Battle of Tannenberg/Battle of Grunwald on the decline of the Teutonic Order, and I am left with a material-drought.
It is a 4,000 words long college level study written in a High school class, and I require at least three books and two articles for my primary sources.
I have already grabbed one book named "Tannenberg 1410: Disaster for the Teutonic Knights" and two other articles regarding the subject, but I am still in need of more sources or credible information. If there is an expert on the subject of the Teutonic Order's history after 1410, I would be delighted to get in contact.
The Teutonic Knights have generally not been as widely covered in English language scholarship as their more famous brethren: The Templars and Hospitallers. That said, there are a few works on them that should be a good starting point for further research.
Tannenberg 14:10 Disaster for the Teutonic Knights is an Osprey book by two individuals who are not experts on the Teutonic Knights so I'd be wary of trusting it too much. It's probably broadly factually correct but lacking in deeper analysis.
William Urban wrote a book on the Tuetonic Knights about a decade ago that currently seems to be the best available English language book on the subject. It's not perfect by any means but it is a good starting point. His bibliography should certainly contain some sources that warrant further investigation.
The author on the Teutonic Knights I'm most familiar with (although I'm only passingly familiar with his work) is Sven Ekdahl. He's published a series of great articles on the order and a few books. The problem is that most of his work is in German. If you read German and can get your hands on his stuff I recommend it. If not, he does have a few good articles published in some Crusading history volumes that concern the military prowess of the Teutonic Order.