LANGUAGE, TRUTH AND LOGIC is the classic work of philosophy by Alfred Jules Ayer published in 1936 when Ayer was 26 (though it was in fact completed by age 25). This book defines, explains, and argues for the verification principle of logical positivism, as it relates to the use of objectives and methods in determining truths and probabilities. And whether or not one agrees that emperical evidence is the only basis for proof, there is no denying that this is a brilliant book in how it explains in what ways the principle of verifiability may be applied to the problems of philosophy itself.
If you can't prove something, it is literally senseless - so argues Ayer in this irreverent and electrifying book. Statements are either true by definition (as in maths), or can be verified by direct experience. Ayer rejected metaphysical claims about god, the absolute, and objective values as completely nonsensical. Ayer was only 24 when he finished LANGUAGE, TRUTH & LOGIC, yet it shook the foundations of Anglo-American philosophy and made its author notorious. It became a classic text, cleared away many cobwebs in philosophical thinking, and has been enormously influential.