About Lutheranism
What does it means to be Lutheran?
Start with the book Lutheranism 101 which covers the basics about Lutheranism.

For an in-depth exploration, consider reading the Book of Concord, focusing on the Augsburg Confession and its Apology (Defense). The Augsburg Confession is the foundational Lutheran document that articulates our beliefs based on the Scriptures.

If you are looking for quick bullet point information, you can start with Luther's Small Catechism. Be aware that the Book of Concord does contain both the small and large catechism text.
You can also read the book "The Spirituality of the Cross" by Gene Edward Veith Jr. which is an easy to read book for new Lutherans and those looking to become Lutheran. You can get it in book, eBook, and find it on Audible.

A good video on what Lutherans believe: