The one essential tool I think those who are seriously interested in the Middle Ages need is a good reference text on Medieval Catholicism. Without a good understanding of the Church and Christendom, there simply cannot be a good understanding of the texts or of the society. Even most contemporary Roman Catholics have forgotten (or never understood) topics such as the medieval concept of sin (mortal, venial, original), monasticism, fasting, the use of psalters, rosaries, breviaries, etc. To fill in the knowledge gaps in these and other areas, I use and strongly recommend this book. I discovered it 8-10 years ago at Cambridge University where it was wildly praised and widely read. It may not be a book you'd want to read straight through (though you could do so because it's not that long), but it is one that you'll want to consult again and again as a reference text.