John Howe, along with several other artists, is a master at his craft. Having been published in books, calendars and what ever else you can find Tolkien related images he is one of the most recognized artists out there.
His website is teaming with information, images with explanations and blog like entries to linger in. Even his links page is a wealth of information. And last but not least the man even as a forum on his site.
To visualize Tolkien's world remember John Howe.