Logic or the Art of Thinking
This book first appeared in 1662 and was written by two monks from the Port-Royal Abbey, a center for Catholic Jansenism. This work is a companion to General and rational Grammar: The Port Royal Grammar which appeared two years before the logic. it has recieved renewd attention largely due to Chomsky claims in Cartesian Linguistics and elsewhere it prefigured modern transformational generative grammar.
For example they realized that the surface structure of a sentence need not mirror its deep structure.
For those interested in logic and semantics this book is of great historical and philosophical interest.