Blues Music
Blues music is one of the most influential music forms of the 20th century and has influenced countless other forms of music such as Rock N Roll, Pop, Hip Hop, and many others. Some popular blues artists include Bessie Smith, Ma Rainey, B.B. King, and Stevie Ray Vaughn to name a few.
As well as blues music blues guitar is one of the most influential instruments of the 20th century, in my opinion it practically gave birth to modern rock guitar. Many of the early blues artists such as B.B King, Robert Johnson, and Albert King had huge influences on the hugely famous rock/blues players of the 60's and 70's such as Jimi Hendrix, Eric Clapton, and Jimmy Page.
With blues music and blues guitar having such a huge impact on modern music and guitar I would highly recommend guitarists and any musicians to study the history of blues music.