I have read all of McKenna's books, as well as Dr. Rick
Strassman's. They are all good-- McKenna's are fun and
interesting to read, as he has many intersting ideas and is a great
storyteller, but his stuff is a bit less pragmatic/scientific and a bit
more far-fetched. Strassman, being a scientist, is very thorough
and precise in his approach, and while the subject matter is
fascinating, some found his book to be a bit "dry", so to speak.
I personally liked all of the above-- "True Hallucinations" and "The
Invisible Landscape" were my favourites by McKenna, followed by "The
Archaic Revival". I also very much enjoy the work of Jonathan
Ott-- he is absolutely brilliant and the intro to "Pharmacotheon",
called the "Proemium", is possibly the best assessment and critique of
drug prohibition and argument for cognitive liberty ever written.
Also, if you haven't read "Tihkal" and "Pihkal" by Alexander and Ann
Shulgin, you must.
Alex Grey also has two books that are amazing, called "Sacred Mirrors", and "Transfigurations".