Various: Albums from the Smithsonian Folkways series (Folkways/Southbound)

The Smithsonian is one of those great American institutions which, if it says "we're here to help" actually is. In their Smithsonian Folkways collection they have short audio examples of 40,000 tracks and through their Global Sound website they are all available for download. And they have the original liner notes for the relevant albums which you can view for free.
Which might make these collections seem a little redundant.

The ones here have been out for some time but a pile of them landed at Elsewhere so here's the shorthand.

The Classic Blues title includes tracks by Sonny Terry and Brownie McGhee, Memphis Slim with Willie Dixon, Son House, Champion Jack Dupree, Reverend Gary Davis and others.

Classic Labor Songs has Paul Robeson, a bunch of Seegers (Pete, Mike and Peggy), Utah Phillips, Woody Guthrie and Joe Glazer.

Classic Railroad Songs has Lead Belly, Doc Watson, Guthrie, Cisco Houston, Pete and Mike Seeger, Elizabeth Cotton and more.

Classic Bluegrass includes Bill Munroe, Red Allen, Doc Watson, the Stanley Brothers, the New Lost City Ramblers, Hazel Dickens . . .

And Classic Piano Blues album weighs in with Memphis Slim, Speckled Red, James P Johnson, Champion Jack Dupree and many others.

These are exceptionally well annotated, pocket-sized colllection of great (Lead Belly, Josh White, Lonnie Johnson) and sometimes overlooked artists (Sammy Price, Roosevelt Sykes, Snuffy Jenkins, Etta Baker) which allows you to fine tune your era to the era and the individuals.

Here too is Lead Belly on piano (he's most often thought of as a guitarist/singer) and Big Chief Ellis who was part Black Creek Indian.

Worthy additions to your library, but also a way into the rather bigger library the Smithsonian has online.