This is an extremely elegant and formally rigorous text (I intentionally refrain from saying 'introduction') in first-order logic. The downside is that the text is pretty mathematically demanding, as Smullyan presumes that the reader is already familiar with formal logic. This book is thus not recommended as a first text in logic, but for those with some formal logic under their belt, this thin volume is a handy reference and an assured guide for digging further. ( )