There is a rich history of jazz vocals, of course, but it's somewhat of a tangent from the main story of the evolution of jazz. As jazz moves forward it's usually instrumentalists who are changing the way the music is played. But, if you want to know who the essential singers in jazz are I'ma tell you right now.
The Men of Swing: Cab Calloway and Billy Eckstine
We've seen how big bands of the swing era would revolve around a charismatic leader like Duke Ellington or Benny Goodman. It was also possible for that front man to be a singer.
Cab Calloway was a dynamic entertainer with a ton of stage presence. In the mid-1940s he would unwittingly help spur the beginning of bebop by firing Dizzy Gillespie from his band.
People my age know him from his appearance in the movie The Blues Brothers
Also, Billy Eckstine (or "Mr. B") was the smooth matinee-idol type singer of the swing era. Perhaps he seems a bit corny today but back in the day he was hot stuff.
Billie Holiday (1915-1959)
It was also common for the big bands to tour with a "girl singer" who would come out and do a few numbers
Billie Holiday rose to prominence during this era, singing with big bands like Benny Goodman and Count Basie and doing her own gigs with smaller bands in bars. Holiday's way of singing stood out from the "girl singer" pack, however, because she delivered her phrases more like a saxophone or trumpet player would - her delivery is always very laid-back and her voice has a very mysterious, sultry tone. In my opinion she's the one jazz singer that you really *have* to listen to, an all-time great.
Here's what she sounded like in 1936 (age 21):
Hard living and substance abuse eventually caught up with Billie and her late work sounds a little different. Here she is in 1957, appearing on "The Sound of Jazz."
Ella Fitzgerald and Sarah Vaughan
With Fitzgerald (1917-1996) and Vaughan (1924-1990) we see singers who come up in the swing era but can really hold their own with the bebop players, singing out long lines with lots of notes and "scat singing" on the same level as the horn players. Both had long, distinguished careers.
Ella is perhaps like the big sister to Sarah, because she was a little older and became a little more famous. Her style of performance is usually very cheerful and high-spirited. Listen to what she does with "Mack the Knife" as it goes along, gradually getting into a Louis Armstrong impression and scatting.
(The song itself is a collaboration between composer Kurt Weill and playwright Bertold Brecht - it tells a story about a serial killer in a straightforward, almost happy way, which is supposed to be chillingly ironic.)
Sarah Vaughan has a little comedy bit worked into her version of "How High The Moon" that kicks off a scat solo:
Frank Sinatra (1915-1998)
Of course the jazz singer that people are most familiar with is Sinatra. He stays in the swing groove for pretty much his entire career and makes a specialty out of presenting jazz tunes in a very masculine, no-nonsense style. His extreme popularity amongst young people in the 1940s inspired the terms "bobby soxer" and "teenybopper," and it anticipated the hysteria we would later see around rock stars like Elvis and the Beatles.
Students have pointed out that he doesn't really do the scat-singing improvisation that other singers like Armstrong, Fitzgerald, and Vaughan do.
Nina Simone (1933-2003)
Nina Simone is a different sort of singer who emerges in the sixties. Her life story is detailed in the excellent netflix documentary What Happened, Miss Simone - as a little girl she trained to become a classical pianist, but turned to jazz and pop performances as a way of making a living. Her music blends jazz, blues, folk and classical into a unique combination and her low alto voice and strong delivery are very striking.
As her career developed she became very interested in the Civil Rights movement and recorded some famous protest songs like "Mississippi Goddamn."
Jeanne Lee (1939-2000)
My favorite "avant-garde" jazz singer is Jeanne Lee, who was part of an interesting piano duo with Ran Blake. Blake would play piano parts that had a wandering, atonal vibe to them and Jeanne's job was to do the melody in a relatively straight-ahead way so Ran could get weird in the background.
Here they are doing a Monk composition called "Worry Later."
Cassandra Wilson (b. 1955)
If you want a jazz singer who is plugged into more recent trends I would pick Cassandra Wilson.
Cécile McLorin Salvant (b. 1989)
...and if you want someone who is active right now, but is kicking it old-school (in a Wynton Marsalis-like way) you need Cécile McLorin Salvant.
Of course there are hundreds more people out there singing jazz and jazz-adjacent styles, these are just the people I would pick as "important." Hope you found this summary interesting.