<bookstore>
<book location="US">A1</book>
<category>
<book location="US">B1</book>
<book location="FIN">B2</book>
</category>
<section>
<book location="FIN">C1</book>
<book location="US">C2</book>
</section>
</bookstore>
So Given the above; you can select the first book with
(//book[@location='US'])[1]
And this will find the first one anywhere that has a location US. [A1]
//book[@location='US']
Would return the node set with all books with location US. [A1,B1,C2]
(//category/book[@location='US'])[1]
Would return the first book location US that exists in a category anywhere in the document. [B1]
(/bookstore//book[@location='US'])[1]
will return the first book with location US that exists anywhere under the root element bookstore; making the /bookstore part redundant really. [A1]
In direct answer:
/bookstore/book[@location='US'][1]
Will return you the first node for book element with location US that is under bookstore [A1]
Incidentally if you wanted, in this example to find the first US book that was not a direct child of bookstore:
(/bookstore/*//book[@location='US'])[1]