Pretty medieval manuscript of the day is a glorious fifteenth century missal from the collections of the National Library of New Zealand. According to the caption on the Flickr Commons:
This Missal was copied in England (perhaps London) between 1400 and 1425, possibly at the workshops of Johannes and of Herman Scheerre. A Missal is a book containing the text for the Mass. This beautifully decorated manuscript contains illuminated initials in varying sizes from 1- to 6-lines. Originally it also had three elaborate, probably historiated, 8-line initials, but two of these have been cut out of the manuscript (one of these pages is shown here on the left). The left-hand page also has an elaborate border with scroll-acanthus, birds, and feathery spray motifs in blue, orange, red, green, white, black and gold. At the fore-edge of the right-hand page you can see a white page-marker.
Image source: Alexander Turnbull Library MSR-18. No known copyright restrictions. Available via the Commons on Flickr.