What is the great  American songbook? Well if we turn to our old friend WIKIPEDIA we discover the following:
The Great American Songbook is a construct that seeks to represent the best American songs of the 20th century
[1][2][3] principally from 
Broadway theatre, 
musical theatre, and 
Hollywood musicals, from the 1920s to 1960, including dozens of songs of enduring popularity. The Great American Songbook became (and remains) a vital part of the repertoire of 
jazz musicians, who describe such songs simply as "
jazz standards".
The early years
Since the 1930s, many singers have explicitly recorded or performed large parts of the Great American Songbook.
Ella Fitzgerald's popular and influential 
Songbook series on 
Verve in the 1950s and 1960s collated 252 songs from the Songbook.
Other influential early interpreters of the Great American Songbook include 
Fred Astaire, 
Shirley Bassey, 
Tony Bennett, 
Pat Boone, 
June Christy, 
Rosemary Clooney, 
Nat "King" Cole, 
Barbara Cook, 
Perry Como, 
Bing Crosby, 
Vic Damone, 
Bobby Darin, 
Sammy Davis, Jr., 
Doris Day, 
Jo Stafford, 
Blossom Dearie, 
The Four Freshmen, 
Judy Garland, 
Eydie Gorme, 
Johnny Hartman, 
Billie Holiday, 
Al Jolson, 
Jack Jones, 
Cleo Laine, 
Frankie Laine, 
Steve Lawrence, 
Peggy Lee, 
Julie London, 
Dean Martin, 
Johnny Mathis, 
Carmen McRae, 
Helen Merrill, 
Wayne Newton, 
Dinah Shore, 
Bobby Short, 
Nina Simone, 
Frank Sinatra, 
Barbra Streisand (particularly in her earlier work), 
Mel Tormé, 
Sarah Vaughan, 
Dinah Washington, and 
Andy Williams.
 Contemporary singers
Over the last several decades, there has been a revival of the Songbook by contemporary singers.
In 1970, 
Ringo Starr released 
Sentimental Journey, an album of 12 standards arranged by various musicians. In 1973, Grammy-winning singer-songwriter 
Harry Nilsson released a critically well-received album of 12 classic standards, 
A Little Touch of Schmilsson in the Night, arranged by 
Gordon Jenkins. The album was re-issued on CD in 1988 with a total of 18 standards sung by Nilsson. Also in 1973, 
Bryan Ferry, of 
Roxy Music fame, released 
These Foolish Things, and he has subsequently recorded several such albums. In 1978, country singer 
Willie Nelson released a collection of popular standards composed by such notables as 
Hoagy Carmichael, 
George Gershwin, and 
Irving Berlin titled 
Stardust. This was considered risky at the time but has become perhaps his most enduring album.
In 1983, popular rock vocalist 
Linda Ronstadt released 
What's New, her first in a trilogy of albums of standards. Stephen Holden of the 
New York Times wrote:
| “ | What's New isn't the first album by a rock singer to pay tribute to the golden age of the pop, but is ... the best and most serious attempt to rehabilitate an idea of pop that Beatlemania and the mass marketing of rock LP's for teen-agers undid in the mid-60s. During the decade prior to Beatlemania, most of the great band singers and crooners of the 40s and 50s codified a half-century of American pop standards on dozens of albums, many of them now long out-of-print.[5] | ” | 
In 1991, 
Natalie Cole released a highly successful album 
Unforgettable... with Love, which spawned a 
Top 40 hit "Unforgettable", a virtual "duet" with her father, 
Nat "King" Cole. Follow-up albums such as 
Take a Look were also successful.
Since the mid 1980s, vocalists such as 
Michael Feinstein, 
Harry Connick, Jr., 
Michael Bublé, 
Diana Krall, 
John Pizzarelli, and 
Ann Hampton Callaway have been notable interpreters of the Songbook throughout their careers. Michael Feinstein in particular has been a dedicated proponent, archivist, revivalist, and preservationist of the material since the late 1970s.
 Other singers
Since 1980, various established singers in unrelated genres have also had success in treating the Songbook. Beginning in 2002, 
Rod Stewart has devoted a series of studio albums to Songbook covers. Other rock and pop artists who have utilised the work include 
Keith Richards, 
Carly Simon, 
Bette Midler, 
Barry Manilow, 
Caetano Veloso, 
Queen Latifah, 
Joni Mitchell, 
Boz Scaggs, 
Robbie Williams, 
Sting, 
Ray Reach, 
Pat Benatar, 
Morrissey, and 
Rufus Wainwright.
John Stevens, a 2004 
American Idol contestant, also gave exposure to this trend. 
Steve Tyrell has forged a successful solo career with his interpretations of songs from the Great American Songbook. His version of "
The Way You Look Tonight" for 
Father of the Bride (1991) was noticed and kept in the film at the insistence of its star, Steve Martin. This led to several albums, including 
A New Standard, 
Standard Time, and 
Bach to Bacharach.