You can find many of them at the Internet Archive:
You can find many of these in facsimile editions on Amazon (just search for "WPA Guide" or "Federal Writers' Project"). But the versions at the Internet Archive are superior in one way: you can grab just the text and edit the hell out of them to fit your game. Here's an example using the Montana guidebook:
Montana is too large and diverse for definition or characterization in general terms. Children in its schools are taught that the name Montana means "mountains," but many of them see only prairies rolling to the horizon. They are told that Montana is still a great ranching State, where cattle graze and cowboys ride, but some of them, as in Butte, see only ore dumps, great dark sheds, and barren buttes.
To the dry-land farmer in the eastern part of the State, Montana is a vast agricultural plain checkered with brownish fallow land and fields of green wheat that ripen to a dusty gray-gold in August; or it is a drab waste seen through a haze of wind-blown soil. For him the mountains of the western part exist chiefly as the goal for some long- defer red vacation.
And here's how I plagiarized and mashed it around to create some background for a minor in-game location:
Monfortia defies definition or general characterization; a small kingdom ruled by an aging monarch, it has seen better days and does not know it is in a death-spiral of irrelevance. Children in its villages and thorps are taught that the name Monfortia means "The Place of Man's Fort," as the land is said to be the cradle of all civilization in the Known World.Many ancient, shining ruins are scattered throughout the lands and, it is said, these ruins hold rich treasures from the dawn of man within their shattered stones. But many of these children see only pasture land or wheatfields rolling to sky's edge. They are told that Monfortia is still a great agricultural power, where aurochs, sheep, and goats graze, and in many places this is true. But most children, as in Slagston and Knife's Edge and Killorcsnau, see only slums, street orphans, ore dumps, endless streams of chained slaves, encampments of men and half-men wearing blood-streaked Monfortia armor and streams diverted to temper the once-famous firesteel still produced below the earth, now with none of its legendary rust resistance.
To the scrub farmers in the eastern part of the kingdom, Monfortia is an endless barren plain dotted with brownish land grown fallow and struggling fields of summerwheat ripening to a dusty amaber at Fallharvest; or it is a suffocating waste of wind-blown soil seeking vengeance during Seasonstorm. For the farmer, the mountains to the western edge of the kingdom exist merely to taunt him to consider a vacation he knows he will never take or to punish him for dreaming his life could be better. Yes, for now when the farmers look to the mountains it is not out of hope, but out of fear. For their king has fallen under the lust-spell of a beautiful beast who slunk down from the crags--and they worry that more may be coming. And there are.
All the guides hosted at the above link are in the public domain if you live in the U.S.--which means you can legally do whatever you want with texts, including changing it and publishing it. Of course, ethically, you should give credit to the original source if you do.