According to Katie on her blog Chasing Vibrance, a devotional is,
Unlike devotions, a devotional is a book meant to help guide your time with God. So in your devotions, you may read a devotional, but you don’t have to. Many devotionals have Scriptures in them and also the author’s explanation of the passage or spiritual encouragement. Many of them last a full year and can be repeated many times over. Some buildd on each other, but many you can hop in at any day and still have the same impact.
You can find a list of the most popular devotions in amazon's best sellers list (we are not an affiliate - so we don't make money off you clicking any links):