What should you learn first?

You need to look a little harder for a beginners rock book. There are some out there. In any case learn the basic open chords that are used the most along with some of the basic minor chords: C A G E D F, a minor, e minor, d minor. some of these chords are used in almost every song you're going to learn eventually.

The actual basics of music theory are the same for every genre, from classical to heavy Metal. Beginners books tend to concentrate on easy music, to give the student confidence - the musical style is really irrelevant. As you progress, you'll find that your natural instinct will take you towards your favourite music, but, by then, you'll have enough knowledge to make the leap much easier.

The less expensive beginners books are loaded up with old blues and traditional folk stuff (at least partly) because that stuff is public domain, and they don't have to pay any cash to the copyright holder.
As has been mentioned, there are "rock for beginners" books out there.

I can't really comment on them, though, as I'm both a beginner, and notoriously cheap.

There are some good beginner's lessons right here on GN that use rock songs.

You might be interested in Total Rock Guitar, by Troy Stetina. http://www.amazon.com/gp/product/0793587875

I have his Metal Rhythm and Metal Lead guitar books. I worked through parts of them until I realized I wasn't actually interested in playing in a metal band :), but his instructional style is pretty good.

If you are interested in metal and rock then you could start with power chords. They are easy enough to learn and you should be able to play songs fairly quickly. BUT do not neglect to practice open chords, barre chords, scales, etc. I think power chords will get you motivated because you'll start to sound good with distortion which will motivate you more to learn more. I may be mistaken, but I believe OneWingedAngel started with powerchords and I remember when she posted her first recording I couldn't believe she had started around the same time as me.

To be a well rounded guitarist you need to practice it all.

Thanks for the responses, all!

I guess part of me feels that I should learn all the stuff to be a well-rounded guitarist, but then part of me wonders whether some of the stuff is necessary if I'm interested in mainly playing rock music. For instance, stuff like fingerstyle or reading standard music notation... they seem like good things to know, but they don't really seem essential for rock guitar.

But on the other hand, I don't want to cut any corners if it's going to catch up to me in the future.

Fingerstyle isn't necessary for R&R, but not a few guitarists use it - Jeff Beck, Mark Knopfler, for example.
The argument over standard notation will run and run and run and..........................
Like fingerstyle, it isn't necessary, there's plenty of tab around. Sadly, plenty of that tab is inaccurate - it is, after all, only one person's interpretation of the song. Also for the vast majority of tab, it is assumed that you already know the song, in detail, because there is no indication of timing. I would suggest that you dedicate a small amount of your practice time to standard notation, you'll appreciate it, in the end.