Finding the First Node That Meets a Condition

Suppose you want to obtain from a collection the first node that meets a certain condition. For example, you want the first book whose author's last name is Bob. You can specify the following query:

(//book[author/last-name="Bob"])[position()=1] 

The following two expressions appear to also return the first book whose author's last name is Bob, but they do not. Instead, these queries both return a book whose author's last name is Bob only if that book is the first book in the document.

//book[author/first-name="Bob"][position()=1] 
//book[author/first-name="Bob" and position() = 1]