Not sure which Maple version you use, but if it's Maple 18, you can simply click the "Programming" link from the start page to get to a bunch of help/tutorial documents which I find quite accessible.

A good book on computer algebra system (CAS) also helps a lot if you want to do serious programming in Maple. I personally like "Intro. to Maple" by Andre Heck.  The Maple part is probably outdated, but his explanation on the structure of CAS is pretty good, and I think it's still very relevant to Maple's nuts and bolts. 

http://www.amazon.com/Introduction-Maple-Andre-HECK/dp/0387002308

MapleNet the product has nothing to do with the said website.  Try http://mobius.maplesoft.com/maplenet/mobius/index.jsp