If the SAT or ACT had a rule book, that book would rest on a stone tablet.  If you were to lift the book from the tablet, you would find a single commandment inscribed there:

Thou shalt read and read and read.

If you really got this image, with a flimsy paper rule book, and the commandment on monolithic stone the size of a giant boulder or maybe even Washington Monument, you'd have it about right.

Everything else in terms of test prep for the ACT and SAT rests on a solid foundation of reading and reading and reading.

One of the main reasons that those courses people pay thousands for to get their kids to perform better on standardized tests don't produce that much of a change is that it takes a considerable amount of reading to improve a score, no matter what, and that can't be done in a test prep class.

In the reading sections of the test, the ability to quickly absorb and dissect a passage is absolutely essential and only obtainable through a high volume of reading.

In the grammar section, if you don't know what "sounds right" in English you'll be lost, no matter how many rules you've memorized the month before the test.

And it goes without saying that a college level vocabulary is a must to truly perform well on the ACT and SAT, and that can only be arrived at through reading.  Flash cards can help, but will only get you so far.

What to read?

Anything, but particularly books AT YOUR READING LEVEL.  Fiction or non-fiction.

Try reading too far above your reading level (which is the level you're able to read comfortably and enjoy what you're reading without straining or getting lost or confused) and you won't be reading for long.  The difficulty will strip the fun out of it, and eventually you'll stop reading.

The whole point is to read a lot.  So why not make it easy on yourself?

What books are you reading now?