Here are my opinions on what I consider to be the best game books on Medieval Life
GURPS Low Tech,
This combined with the e23 supplements is by far the most useful gaming supplement on pre-industrial life. Since with these types of books GURPS is focused on realistic emulation the few mechanics can be easily translated.
The length is not due to in-depth exploration of any one topic but rather due to is comprehensive sweep of pre-industrial technology and society. The hardback is focused on technology and the three e23 release on pre-industrial society and life And it has a extensive bibiliography.
The downside is that the mechanics are in my opinion too abstract even for GURPS and really a toolkit for the referee to come up his own figures. And it is oriented to a single RPG.
A Magical Medieval Society: Western Europe
For the d20 world this is one of the best supplement. It is comprehensive and well written. The downside is that the mechanics are just OK, and it is a bit wordy.
I also recommend its companion A Silk Road which discuss medieval trade focusing on long distance trade. It include material I haven't seen written anywhere else for Roleplaying Games.
Fief and Town by Lisa J. Steele
A good solid writeup of medieval life on the manor and in the town. A bit pricey but if you like reading the writing style of the preview, this will be a useful purchase.
Various Harn Articles
The Harn Campaign setting is organized into a series of articles some of them on Medieval life and not specific to Harn at all. The downside they are pricey, upside is that what game mechanic are straight forward and easy to use with other RPGs and they are concise.
I recommend particularly Harn Manor
It hits a sweet spot in terms of emulating manorial life, in the way that Classic Traveller hits a sweet spot in starship construction and interstellar trade. Yes it involves some math and a spreadsheet but it all straight forward. But what makes it shine is how it generate the yearly issues that the manorial lord has to deal with. Basically each tenant household rolls on a chart and some of the results can lead to adventures or complex resolutions for the lord.
And if you can find it Harn's Pilot Almanac has stellar Ship Construction rules and a straight forward medieval trading system. Again we are talking Classic Traveller simplicity.
Ars Magica, a Medieval Handbook.
This is similar to Lisa Steele's Fief and Town, it is a more comprehensive and a lot cheaper, but devoid for the most part of Game mechanics.
These represent the what I consider the best and most useful summaries of Medieval life for gaming. I own all of these (and others) and used many of them in my Majestic Wilderlands campaign.