This page may contain just about all you need to know about maintaining a healthy diet. You've got to scroll down quite a bit to get to the actual recommendations. Once you read them, though, I'd recommend going back up to the second paragraph and clicking on the link for the full guidelines. The report is about 80 pages, but it's split up so you can just read the parts you have questions about. The second link on the page is broken.

There is a lot of good information on this page, but I'd skip the first few paragraphs and start reading after the "Adequate nutrients within calorie needs" heading. Click on the link at the top of the page if you want more detail, it'll take you to the complete report.