Who is the author of Angel of the Dark?

Sheldon is the author who originated the series and characters. Bagshawe was hired to write this book after the author's death. Bagshawe cannot be given full credit for a book that is not truly hers - she doesn't own the copyright because she's only a hired writer who was brought on to finish the series after the original writer died, which is why there are two names listed as the author. Bagshawe can't be listed as sole author of a book that's not hers, she's considered a co-author.

To answer your question: both are the authors of the book.

This situation is not uncommon after a successful author's death. Sometimes the hired author is not named as a cowriter and the only name that appears on the book is the deceased author's. And, sometimes a deceased author's book will have several writers over a span of books, but all the books might only list the deceased author as the only author of the book on the cover.

HTH.