I found some free (etude) books for cornet/trumpet on the web:
Bousquet “36 Celebrated Studies for the cornet”
Edwin Franko Goldman “Practical studies for the cornet (and trumpet)”
Albert G. Mitchel “ (1920)”
Gabriel Parès ” Daily exercises and scales : Bb trumpet (or cornet)”
Adam Seth Mitchel “An Introduction to Baroque Ornamentation for Solo Trumpet Literature”