Brian Tierney, Western Europe in the Middle Ages 300-1475.

One of the best texts on the period, period. Very readable, organized by theme and location. Heavily focused on (what would be) France, Germany, England and Italy, but the coverage of peripheral areas is of quality, if not terribly extensive.

Einhard, Notker, and Ganz (editor/translator), Two Lives of Charlemagne.

Biographies of one of the most important men of the earlier period, from one who knew him and one writing 50 years after his death. Very interesting changes in perspective and characterization, and gives great insight on concepts of rulership of the time.

Gregory of Tours, Thorpe (translator), A History of the Franks.

While at times it's a bit of a slog, this is a very rewarding text - queens and princesses trying to kill each other, rebel nuns rallying miscreants into armed mobs, Clovis and his warband described as 'lusty freebooters' (Thorpe's translation is delightful) - the list of bad behavior goes on and on. Granted, this is a late 6th century text, but it's superb for flavor, and it does give a starting point to understand the culture of the Medieval period.

All of these are available cheap on Amazon as used textbooks, and from my experience, a lot of those will be in near perfect condition...

ETA:

Madden, New Concise History of the Crusades.

Solid, balanced work on primarily the Eastern Crusades. Does a very good job of dispelling much of the popular 'wisdom' on the subject, and placing the Crusades in context.

Drew, The Lombard Laws

Very, very illuminating work on Lombard legal codes of 6th to 8th centuries - what they punished and how is eye-opening.