Georg Henrik Von Wright 
Logical Studies [PDF ebook] 

First published in 2000. In this volume are eight essays; with the first three essays deal with the problem of logical truth. Their aim is to elucidate what is meant by saying that logical truth is formal-dependent of form and independent of content-or that logical truth is tautologous. The next is on study of distributive normal forms that awakened my interest in modality. The next three essays are in the field of modal logic. Related to modal logic are the problems of the conditional (the if-then) and of entailment (logical consequence) on which the final essay is based.