XDocument doc = XDocument.Load(@"catalog.xml");
            IEnumerable<XElement> bookItems =
doc.Root.Element("books").Elements("item");
            IEnumerable<XElement> cdItems =
doc.Root.Element("cds").Elements("item");

That simply selects the 'item' XElement objects. If you want to populate
some anonymous type then do it like this:

            XDocument doc = XDocument.Load(@"..\..\XMLFile1.xml");

            var query =
                from item in doc.Root.Element("books").Elements("item")
                select new { key = (string)item.Attribute("key"), value
= (string)item.Attribute("value") };

            foreach (var item in query)
            {
                Console.WriteLine("{0}: {1}.", item.key, item.value);
            }