Investment
Collecting stamps shouldn't be about the investment. Many of the stamps collected are worth only what a collector paid for them. A few stamps in your collection may be worth more then you assume from their purchase price or appearance.
Online Stamp Catalogs
Websites exist that catalog stamp collections. These sites are common and provide a good and timely overview of the stamp collection world. Sites such as StampCatalogue.org provide pictures and the history behind items. These sites often list the stamps' prices, depending on quality.
Offline Stamp Catalogs
The biggest book of stamp collecting, and a good reference for collectors, is Scott Postage Stamp Catalog. It shows pictures of each stamp, and lists the country of origin and price, based on the stamp's quality. Also subscribe to magazines specializing in stamp collection. They list each stamp's value in a timely manner and provide news about your hobby.