iA Writer is the writing app in the Ulysses guide. It is a markdown editor built around a calm page and plain-text files. The Mac app is a one-time purchase, sold separately from the iPhone, iPad, and Windows versions. The developer's buy page listed the Mac app at $49.99. Confirm the current price on ia.net before you buy.
Productivity
iA Writer
A focused markdown editor sold as a one-time purchase on the Mac.
Paid⚡ Productivity