I'm absolutely delighted every time I find a new manuscript fully digitised and freely accessible online - it makes research so much easier, and really helps to get a better overview of what was used and available in the Middle Ages.
Yes, not every book has illuminations, and of course not everything is shown in manuscripts, and also of course one needs to take what is shown with a grain of salt, or sometimes maybe even a teaspoon of salt. But still - the more different images you get to look at, the more chances you have at seeing something for the first time, or seeing something way clearer than before.
That all said, here's one of the richly illuminated books, online for your pleasure: St. Elisabeth's Psalter, made around 1201-1208. Click on the eye on the book at the right edge to leaf through the book.