What was the first book Charles dickens wrote and why?

Sketches by Boz and The Pickwick Papers.

Dickens began publishing his Sketches by Boz, a collection of written pieces in the press. The following month (,The first installment of The Pickwick Papers is published on March 30, 1836) he publishes the first installment of the serial novel The Pickwick Papers, which ran until November 1937