antartic press as said above has stuff but mainly from them i find them haveing the how to color for comics best. its pretty extensive withphotoshop stuff. they do more of the teachings of manga style but i dont like the way tehy do it much.
most of the times artists usually direct you to anatomy books and pose books wich help alot.
if you go to j scott campbells site he talks about his techniques where he started and books he used.
also one good thing is to go to the comic book stores and pick up their sketch books. alot of times they have notes in them or the methods they use.
one really good one is the danger girl sketch book.
also other artists have sketchbooks like jim lee and marc silvestri.
also if you pick up certain comic books like fathom or others that are issue 1 or 0, have the perliminaries in them and sometimes a few pages.
sketch and draw magazines aren’t bad just more of a showcase of pencils more than a technique.
i ould go to artists sites and sometimes their fansites bc one or the other have ifo on them.
also look at what artists you want to get info on b/c some have more than others.
oh yea the j scott campbell site has a drawing jam where he hosts and you can see some pics he works on and the steps he goes through. a whole thread about his techniques.