Doors of Perception is okay if you haven't read anything else on the subject. (If you think about it in historical context, it totally rocks, but that doesn't help readability much, does it?) Fortunately, the current paperback edition of 'doors' includes Heaven and Hell, which is much more intellectually stimulating--providing a very nice scientific/cultural/antropological examination of the trancendental without robbing it of meaning.

Moving forward, doors/heaven make a very nice pair--definitely better than either book alone.