As mentioned earlier, your most important source of information as a singer is recordings. Singing is an aural art, so you need to do a lot of listening to get all the subtleties of phrasing, articulations and vocal inflections available to you as a singer. Listen every day to albums which feature great singers.
All music is an extension of what has come before it, so you need to be aware of the development of singing throughout the history of music. Studying Classical harmony will give you a solid grounding in ear training and music theory which can be used in any style of music. There is also a whole tradition of Classical singing, from the songs of Schubert and Schumann to the Choral music of J.S. Bach, to the operas of Mozart, Verdi and Puccini.
For more recent styles such as Jazz, Blues and Rock, it is essential to listen to a wide variety of singers, some of whom are listed here. There is a lot of crossover between styles which come under the headings of Rock, Pop, Jazz, Blues, R&B and Funk. There are many great singers in these styles, but the following list is a good start.
Blues:
Bessie Smith, Ma Rainey, Robert Johnson, Son House, Howlin’ Wolf, Muddy Waters, Magic Sam, Bobby Bland, BB King, Jimmy Witherspoon, Koko Taylor, Ruth Brown, Etta James, Robert Cray, John Lee Hooker and Lightnin’ Hopkins.
Soul/Gospel:
Mahalia Jackson, Aretha Franklin, The Five Blind Boys of Alabama, Ray Charles, Johnny Adams, Wilson Pickett, Otis Redding, Patti LaBelle, O.V Wright, Ann Peebles, The Jackson Five, The Neville Brothers, Earth, Wind and Fire, James Brown and Stevie Wonder.
Rock/Pop:
Elvis Presley, Little Richard, The Beatles, The Beach Boys (great harmonies), The Bee Gees, Joe Cocker, Janis Joplin, Led Zeppelin, Pink Floyd, Lynyrd Skynyrd, Atlanta Rhythm Section, Little Feat, Steely Dan, ACDC, Van Halen, Freddie Mercury (Queen), Jeff Buckley, Bruce Springsteen, The Eagles, The Doobie Brothers, Sting, George Michael, Madonna, Prince, Celine Dion and Whitney Houston.
Jazz:
Louis Armstrong, Billie Holiday, Big Joe Turner, Ella Fitzgerald, Frank Sinatra, Sarah Vaughan, Jon Hendricks, Bobby McFerrin, Mel Torme’, Tony Bennett, Diana Krall and Cassandra Wilson.
Country:
Patsy Cline, Hank Williams, Merle Haggard, Loretta Lynn, Johnny Cash, Dolly Parton, Charley Pride, Willie Nelson, Emmylou Harris, The Amazing Rhythm Aces, Alison Kraus/Union Station, Don Williams, Dwight Yoakam, Tammy Wynette, and George Jones.
Folk/Acoustic:
The Weavers, Harry Belafonte, Peter, Paul and Mary, Joan Baez, Indigo Girls, Christy Moore, Planxty, The Chieftains, The Dubliners, Enya, Steeleye Span, The Roches, Tom Paxton, James Taylor, Carole King, Crosby, Stills, Nash and Young, Cat Stevens, Jackson Browne, Joni Mitchell, K.D. Lang, Edith Piaf.
There are also many great exponents of World music and various ethnic folk traditions which are not mentioned here. Ask other singers who they listen to get some ideas.