They have
many layers of meaning. I wish I remembered what those instruments in the right wing of
Garden of Earthly Delights meant. Also, I looked at my notes and it turns out Bosch not only didn't use underpaintings... he didn't even sketch before hand! He just had the entire scene laid out in his head before starting the work.
If you like Bosch's style, you might also like Pieter Bruegel the Elder. He was influenced heavily by Bosch, but also by the Limbourg brothers (you can search for their Book of Hours, which was never finished because both they and the patron died before completion... plague).
Here's a work I found by Bruegel, though the size here doesn't nearly do it justice. Note the relative lack of overlapping present in Italian works of this period. The northern artists were less concerned with strictly delineated linear perspective than with cramming in as much detail as possible. These works in general are fairly small (Bosch's
Garden of Earthly Delights is (I believe) only like 6'x6' which is very small for the amount of detail.
Also,
Artcyclopedia is a pretty good site for finding a range of online galleries and museums with more information than the Google image search is likely to provide (though that one is great too).
Here's Bruegel's
The Fight Between Carnival and Lent: