Should commas be used around by and an author’s name after a title?
It depends. In the following example, commas are used to set off the by phrase because the phrase is not integral to the meaning of the sentence:
Life after Life, by Kate Atkinson, won several book awards.
If you remove the phrase, the meaning is the same:
Life after Life won several book awards.
But in the example below, no commas are used around the by phrases because the authors’ names are needed to distinguish works with the same title:
I am reading Life after Life by Jill McCorkle, not Life after Life by Kate Atkinson.
If you remove the by phrases, the sentence is nonsensical:
I am reading Life after Life, not Life after Life.