Well the first thing that I would recommend checking out is HackMew's Knowledge. His introduction to ASM lesson was very helpful. One thing you should know, though, is that you should read every single word carefully. If you skim over certain parts, you'll end up extremely confused. The key to learning ASM (or anything, for that matter) is laying down a solid understanding of the basic concepts first. So make sure to read everything and as you're reading, if you don't understand something, read it again.
Next, I would read over JPAN's ASM tutorial/document. HackMew's tutorial is nice and all but JPAN actually delves into a lot of the commonly used opcodes and explains a lot of concepts which you will need to do anything beyond simply moving data around.
After that, I guess you could return to HackMew's Knowledge and read his second ASM tutorial (it's on one of the later pages). I don't think I picked up a lot of new ASM coding information there but I learned a few techniques for hacking in general which will help if you plan on using ASM to change preexisting code (as opposed to simply adding your own code to free space).
Then you'd pretty much be where I am^^ I'm definitely still a beginner so I probably won't be able to teach you much but hopefully those documents can get you on the right track. Oh and as a general resource, you might want to bookmark this page. It has everything you need to know about programming for the GBA and then some. It might be useful if you plan on trying something a little more complex.
P.S. As much as I enjoy being called cool, my name is "colcolstyles" ^^
I notcied that you've been learning quite a bit of ASM, and I've been trying to find some documents to get me started. As far as I know, you're still sort of a beginner at it, correct? So if you have some useful documents and things that will help me get started, can you link them to me?