Without the commas, "by William A. Sabin" is identifying (as if it is part of the name). I'm talking about by William A. Sabin, not The Gregg Reference Manual by Joseph E Smith.My favorite grammar reference book is The Gregg Reference Manual, by William A. Sabin.
The Gregg Reference Manual, by William A. Sabin, is my favorite grammar reference book.
Comma(s) needed in those?
In the second sentence, the phrase "by Brenda Lee" is clearly a parenthetic insertion, hence the two commas. That makes it non-identifying, just adding extra information. You could not omit one comma without also omitting the other.I loved the song “I’m Sorry,” by Brenda Lee.
“I’m Sorry,” by Brenda Lee, was my favorite.