I think one of the best places to start with Luther is the Small Catechism.
The Small Catechism He wrote it to help fathers teach their families basic Christian (Lutheran) principles. It's an easy read because it's designed for home use, and if you want more detail on any of what he says you can look up the same section (Ten Commandments, Creed, Baptism, etc.) in his Large Catechism, which goes into much more depth.
One of his early seminal works was "The Freedom of the Christian", (
https://static1.squarespace.com/sta...+Martin+Luther+The+Freedom+of+a+Christian.pdf) which led to his much more involved "Bondage of the Will". I've read both, and Bondage is an amazing piece. there's a good reason it's considered his magnum opus. That said, it's a deep read. It's not something to dash off in a weekend.
The "95 Theses" are interesting primarily as a historical touchpoint to show the main issues that he had with current (at his time) Roman Catholic teaching. It was an invitation to discussion, not so much an exploration of the issues in detail. It gets pretty redundant after a while.
Probably the closest thing we have to a systematic theology from him is the Augsburg Confession.
The Augsburg Confession He was asked by the Emperor to put in writing his beliefs so the Roman church could dispute them (very general description of the history there... it was a lot more complicated than that). So you have articles "of God", "of the Ministry", "of Justification", etc. Keep in mind when you read them the context here. He will say "this is what we believe, over against what the Romans or the Anabaptists believe".
If you're still reading and want more, the next one would be The Defense of the Augsburg Confession. (also available at bookofconcord.org) Basically, the Roman Catholics replied to his Augsburg Confession (above) and the Defense is his rebuttal. So you get more history within the theology as well.