I use Photoshop Elements, another Adobe product, which you can still buy outright. I just bought a more recent copy from BestBuy for 100, had the disc sent to me, but took the option to download (still can use the disc if needed).
Much easier to learn than Photoshop, and very capable. I am an artist as well as content publisher, and I use a combo of PSE, and PowerPoint, which you can also still buy a year old version outright, or subscribe to the cloud option.
PowerPoint enables you to create 3D covers as well as many other designs. For me, the combination of the 2, has been very helpful. I have never used the software editors that Daniel mentioned.
Plan Your Life In Color!