Prawitz's theories form the basis of intuitionistic type theory, and his inversion principle constitutes the foundation of most modern accounts of proof-theoretic semantics. The proof-theoretical system represents a simpler and more illuminating method than alternative approaches, and this volume offers a succinct, coherent illustration of its applications to natural deduction. 1965 edition.
A concise yet thorough introduction to natural deduction, noteworthy if only for how it treats deductions as "trees", discharging assumptions as functions on said trees, etc.