Illustrated manuscripts aren’t unique to the Middle Ages, but the era really does own the style. Many manuscripts are real works of art, embellished with gold leaf and paints with unusual ingredients. (You can see how scholars are using spectrometry to identify those materials at the MINIARE Project.)
Luckily, the internet means you can see many of these beautiful books without having to trek around the world.
The Parker Library at Corpus Christi, Cambridge has digital images of its collection at http://parkerweb.stanford.edu, including Ms 286, the sixth century evangeliary used in the enthronement of the Archbishop of Canterbury (I can’t link directly to it, but folio 129 verso is very nice), and Ms 61: a copy of Chaucer’s Troilus and Cressida whose frontispiece (fol. 1 verso) may show the poet himself reading the the poem (he’s the man in the center in the red box).
Oxford University has put up a collection of its medieval manuscripts at http://image.ox.ac.uk, including beast-headed evangelists like the dog-headed Mark in this collection of gospels (go to fol. 71v).
The British Library’s digitized manuscripts are at http://www.bl.uk/manuscripts, and include a drawing of the elephant Louis XI of France gave to Henry III of England as a gift. (The Parker has its own drawing of the same elephant.)
Those three sites are really only the tip of the iceberg. The Vatican Library has an ongoing digitization project and there’s an excellent digitized collection at the Bavarian State Library (English-language site here).
Of course, to do more than just enjoy the pictures you need to read Latin. But if you’re really keen the National Archives of the UK have both Latin and paleography (old handwriting) tutorials here.
It’s a Feudal, Feudal World: A Different Medieval History is available for pre-order from Amazon.ca and your local bookstore, to be released in July 2013.