Psalms are songs of praise, and 150 (or 151!) of them make up a single book in the Old Testament of the Bible, and one which was often recited by monks and nuns throughout the week. Like books of hours, books of psalms (psalters) could be personal objects but unlike books of hours, psalters were also used in church services or by individuals in a professional capacity.
“Glimpses of Medieval Life”: The Luttrell Psalter
This project by the British Library places selected images online in very high resolution with a good deal of background information. In particular, the Luttrell Psalter is illustrated with figures of every day medieval life; since we normally see aristocratic fashions (just as you would looking in expensive magazines today) this is very unusual and valuable to people who want to know what it was like to live in the Middle Ages.
This site is a good resource to find images of psalters, but since there are so many the link will take you to the search page. Type in “psalter” in the “Basic Search: Search all” box and see what you come up with!
Codices Electronici Sangallenses
The Library of Saint Galen is another excellent resource; use their search function to find psalters that have been put online. The images and descriptions are of good quality and this library has the advantage of having many completely digitalized manuscripts.
Parallel Latin/English Psalter
This psalter is one that is purely hypertext, but remains an excellent resource because Jerome’s Vulgate text of the Bible was the standard translation from Greek, Aramaic, and Hebrew during the Middle Ages. If you have trouble looking at a page of a manuscript, sometimes it is useful to look at a standard text to try and make sense of the script before you.