The British Library has a number of articles on their website which give an excellent brief introduction to aspects of medieval life through the window of illuminated manuscripts. In particular, see:
Literature, music and illuminated manuscripts
Death and the afterlife: how dying affected the living
Church in the Middle Ages: from dedication to dissent
Many libraries have made available complete digital reproductions of the medieval manuscripts in their collections, and leafing through them can give a wonderful insight into how people in the Middle Ages used different kinds of books in their spiritual lives. For instance, take a look at:
This beautiful Anglo-Saxon gospel-book
A little book of prayers which belonged to an Anglo-Saxon monk
The story of St Edmund of East Anglia, told through a series of full-colour illustrations
A lavish 15th-century Book of Hours owned by Elizabeth of York, wife of Henry VII
Monastic and clerical life
Benedictines, Franciscans, Gilbertines and so many more… A three-part introduction to the different orders of monks, canons and friars in the medieval church
Introducing Medieval Christianity 