Gerhard Gentzen, Investigations into Logical Deduction, in M. E. Szabo (ed.), The Collected Papers of Gerhard Gentzen, Studies in Logic and the Foundations of Mathematics 55, Springer (1969) 68-131 [ISBN:978-0-444-53419-4, pdf]
See also:
Gerhard Gentzen, Über die Existenz unabhängiger Axiomensysteme zu unendlichen Satzsystemen, Math. Ann. 107 (1933) pp. 329-350. (gdz)