Sorry for the double post. But I wanted this one to get your attention.
That book (Understanding Variable Stars) is the ONLY book you will ever need for this event. Seriously. Absolutely everything covered in the rules will be in there except SOME of the DSOs. But it will even have probably 1/4 of the DSOs in the for you too. If you can, just photocopy the whole freaking thing, put it in a binder and take it to competition. But just about all the math, all the background, all the anything will be there. It's a college level textbook, so it goes into the depth necessary for the event, but keeps it (mostly) simple enough that you're not overloaded.
I had not studied for the Astronomy event hardly at all since before the state competition in March last year. On the bus ride down to nationals in May, I read that book, took notes and brought those notes into competition. Got me 10th place. I'm not kidding when I say it is far and away the best resource you'll ever find for this event. I managed to track down a copy at Valparaiso University, so I didn't buy it, but it might be worth buying if you really like Astronomy.