As an author, you may be doing a lot more marketing than you expected, and it can get complicated. You want an ad that will not get cropped in your Facebook and Twitter timelines:
And a square version that will work for Pinterest, Instagram, and WordPress features:
You may want one version for your blog and mailing list:
And a version with less text that can be boosted on Facebook:
And then there are your account banners, which come in different sizes on each platform:
Contact me with your requirements!