tenses - Past Perfect Confusion

In a book these two forms are acceptable:

Before a complete version of Louisa May Alcott's novel Little Women appeared in 1880, the book had been published in two separate volumes.

AND

Before a complete version of Louisa May Alcott's novel Little Women appeared in 1880, the book was published in two separate volumes.

I do not quite understand why both are acceptable. However I decided to just follow it. So I started working on other problems for practice and this is where all the confusion stems from.

By the time Pearl S. Buck was awarded the Nobel Prize for Literature in 1938, she was a best-selling author in the United States for nearly a decade.

So using the general rule for Past Perfect, I said had been. And that, it turns out, is correct. However, since I was bored, I thought: Why cannot it be was? Since the rule provided above accepts either the Simple Past or Past Perfect?

So, is there anyone who can confirm why, for the first example both the Simple Past & Past Perfect can be used without error? And also, why when we come to the second example "was" does not work?

Thank you for your help.