Well, 1. f4 d5 is the Bird opening, so you might find out something about 2.a4 in a book on the Bird, possibly under the header "Second move alternatives that make no sense but knock yourself out".
There might also be books at least partially on 1.a4 and other unorthodox first moves, but since your move order transposes into a strange kind of Bird and there's more Bird literature that would make more sense.
Or you go study some proper openings instead of some random thing that worked against a computer once.
I've just played this opening against my computer (dreamer engine at really easy level 3). It was an experiment to see if I could defeat it more or less easily than usual (I usually set easy level 3 for this kind of experiments). Well, I won way more easily despite the fact my opening was quite a weak one. I mean, I usually win against it at easy level 3, but I'm not a strong chess player by any means and it happens I loose if I choose a opening I do not know how to develop.
So I wonder if I defeated it without a problem because my opening is not included in its book or because it is actually a playable one.
Do you know of any literature about it?