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What's the proper way to use the node sequence (<< and >>) operators?

This is my XQuery:

let $referenceBook := /books/book[2]
let $book := /books/book
where $book/title << $referenceBook
return $book

This looks ok to me, but when I run it, I get the following error. Can you tell me why?

[DataDirect][XQuery][err:XPTY0004]A sequence of more than one item is not allowed as the first operand of '<<' (<title/>, <title/>, ...)

The "node before" (<<) and "node after" (>>) operators can only compare the document position of a single node to the position of another node. In your case, the let $book := /books/book instruction is assigning all the <book> elements in the input document to the $book variable, which means that $book/title corresponds to a sequence of many elements.

Perhaps what you mean to do is this:

let $referenceBook := /books/book[2]
for $book in /books/book
where $book/title << $referenceBook
return $book

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