I read Poor Wizard's Almanac for the first time more than 20 years ago.
It was the first sourcebook I had ever read and it gave me marvelous feelings.
In a small book, I had a world (actually it was only a continent but it's enough for a fantasy world) ready at my feat with many different realms and nations as background for my campaigns and adventures.
The general overview of the various nations was small but gave me a lot of hints to develop my own places and NPCs. The history section satisfied my needs as a little historian while flora&fauna one guided me to create reasonable random encounter tables.
I liked a lot the "Armies of the World" chapter. For a DM like me, who is fond of adventures about politcal struggles and huge scenarios of war, it was very precious. Armies are with the battle rules described in the old Companion Box Set, but you can still use them with more modern sets of rules.
The book also contained a "Current Events" section: a full year of events, from recurrent festivities to world-shaking facts, with also explained what they were really meaning (what was the cause of something or who was really behind it) and how PCs could get involved.
To summarize, Poor Wizard's Almanac gave me access to a real world, alive, ever changing and mutating under my own eyes.
It was a superb sourcebook and still it is nowadays.
There are 3 of them, first two (detailing AC 1010 and 1011) in BECMI (OD&D) rules, while the last one (AC 1012) in AD&D second edition rules.
Since you can easily convert them in D&D 5th edition rules, if you're looking for an exciting and living world, as Mystara is, grab a copy or at least buy it as a Pdf.
You'll be really satisfied.