Evaluating XPath Expresssions

XPath Result of XPath Evaluation
//Book[not(Author = preceding::Book/Author)]
<Book>
                <Title>Illusions The Adventures of a Reluctant Messiah</Title>
                <Author>Richard Bach</Author>
                <Date>1977</Date>
                <ISBN>0-440-34319-4</ISBN>
                <Publisher>Dell Publishing Co.</Publisher>
        </Book>
<Book>
                <Title>The First and Last Freedom</Title>
                <Author>J. Krishnamurti</Author>
                <Date>1954</Date>
                <ISBN>0-06-064831-7</ISBN>
                <Publisher>Harper & Row</Publisher>
        </Book>

The XML Document

<BookStore>
        <Book>
                <Title>Illusions The Adventures of a Reluctant Messiah</Title>
                <Author>Richard Bach</Author>
                <Date>1977</Date>
                <ISBN>0-440-34319-4</ISBN>
                <Publisher>Dell Publishing Co.</Publisher>
        </Book>
        <Book>
                <Title>The First and Last Freedom</Title>
                <Author>J. Krishnamurti</Author>
                <Date>1954</Date>
                <ISBN>0-06-064831-7</ISBN>
                <Publisher>Harper & Row</Publisher>
        </Book>
        <Book>
                <Title>Jonathan Livingston Seagul</Title>
                <Author>Richard Bach</Author>
                <Date>1970</Date>
                <ISBN>0-684-84684-5</ISBN>
                <Publisher>Simon & Schuster</Publisher>
        </Book>
</BookStore>