Science and Sanity is his main work.
But be warned, reading it is likely to take more time than the average book.
Any idea if this Kindle book is an acceptable substitute for the unabridged version? It appears to be a third of the length, but I wanted to read this on an airplane and historically 900 page books have been moderately troublesome for that.
http://esr.ibiblio.org/?p=6549