Although not everyone likes
Daniel Wallace's Greek Grammar Beyond the Basics, I'd say it's the best you can spend your money on, if you want to buy something. Rijksbaron is more difficult (technical and academic) and will probably gather dust in your shelf. I own both. Wallace writes clearly, is easy and light to read and is still after 15 years unsurpassed in explaining aspect in a way which is accurate enough and still easy to understand. And he's correct in his view on time/aspect debate, unlike Decker's paper (in my not-so-humble opinion).
I have also found
Con Campbell's Basics of Verbal Aspect in Biblical Greek useful. It's basically quite like Decker's paper in what it says (for example about time/aspect debate, aspect/Actionsart separation) but is more thorough and easy to read. But the name is misleading because it's actually "Basics of Campbell's view on Verbal Aspect in Biblical Greek", so it's not the lowest common denominator for beginners. You can find some critique on Campbell's book by searching for "Campbell verbal aspect b-greek".
And for the whole time/aspect debate you should search for "verbal aspect b-greek", because there have been quite much discussion about the subject, mostly in the old mailing list. Much of the discussion may be above your level, but in any case you should read something easy and something difficult, and after learning something, read them again. There's no one perfect text or explanation. Rijksbaron, Wallace and Campbell are all good starters, and each has drawbacks. For good balance between easiness and technical correctness I recommend Wallace, while Rijksbaron may be linguistically the most correct but less readable.