Lately the best and fastest source to find other titles in a book series, in my opinion, is Wikipedia (of all places).
Sad to say that Amazon.com continues to disappoint me in this area. It seems to me that Amazon with all their wonderful web features should have by now, found a way to link titles in a book series together.
We need a clear listing of each title with the series name and telling which book number in the series the book is.
Sometimes you can find this information on a book author's website, if they have one. Not all authors have them, especially deceased authors or those who are written under a pen name or by multiple authors (i.e. The Hardy Boys and other children's series).
If you can find an author's website, sometimes the site design is so heavily laden with fancy graphics that just getting to the content is time consuming. Other times the site is pretty but does not have links to quickly find the information. Wikipedia is fast-loading and clear.
An example of a list of books in a series with detailed information is the children's series: Wikipedia entry for The Boxcar Children.
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