Perhaps the best overview of the Thirty Years War would be Peter H Wilson's The Thirty Years War: Europe’s Tragedy. He covers the context prior to the war, the conduct of the war itself, and especially the peace negotiations at the end. Wilson is a well regarded scholar for the Holy Roman Empire in English, and so this should be a good starting point.

Another option--though a fair bit older--is C.V. Wedgwood's The Thirty Years War. While widely considered a classic, it was originally published in the 1930s, and scholarship has advanced in the 90 years since. It could be perhaps a fun exercise to read both and note the differences that emerge between them.