Janet Backhouse was a leading authority in this field (unfortunately she left us in 2004), but her many works about the illuminated manuscript are and will always be available to us.
This introduction is probably the best book to start if you are interested in delving into the beauty of the illuminated manuscript. Not only a gallery, but also an historical overview on how they were made, who made them (not only monks), what themes they covered, who bought them, what resources were needed, what happened with the introduction of printed books, etc.
Some limitations exist: many pages are in black and white, losing considerable impact and all the displayed manuscripts are from the British Library Houses (although there are examples from almost the entire Europe, there are some absences like Scandinavian or early medieval Spanish works...obviously in an introduction we can't expect to have full coverage).
The comments on each plate focus not only on the author (if known), the production place, the style and theme. You will get a fine step stone for further studies.