Invalid XPath expression errors

I get "Invalid XPath expression errors" when I use the expressions:

results = document.selectNodes("/books/book[(details/(price!='100'))]"); or
results = document.selectNodes("/books/book[(details/not(price='100'))]");

But both expressions work OK when I use XMLSPY.

<?xml version="1.0" encoding="iso-8859-1"?>
<books>
<book isbn="11111111">
<title>Das Silmarillion</title>
<details>
<price>100</price>
<author>miller</author>
</details>
</book>
<book isbn="22222222">
<title>Die Belgariad-Saga</title>
<details>
<price>200</price>
<author>shaw</author>
</details>
</book>
<book isbn="33333333">
<title>BookWithNoPrice</title>
<details>
<author>shaw</author>
</details>
</book>
</books>

Regards Mario Mueller