To add to both of these thorough answers, I'd like to mention the problems of textbook adoption. Remember, you're dealing with publishers whose first priorities are to run a business & make a profit- checking facts or correcting inaccuracies often comes second. Often, lobbyists involve themselves in the adoption process, essentially "vetting" each text and rejecting those with which they don't agree. This may include people from many different groups and philosophies, all fighting to ensure their interests are met over those of the others.
Also, if you're disappointed with your textbook, I'd recommend James Loewen's amazing little book Lies My Teacher Told Me. In it, the author surveys 12 different high school textbooks, including those designed for advanced courses. But instead of just pointing out inaccuracies & relating the actual facts, he researches the purpose behind omissions and changes, creating a fascinating look at how history is interpreted in schools.
One last note: you may want to look into Howard Zinn's A People's History of the United States. It really is an incredible read. While many will argue this point with me, since Zinn is an outspoken liberal revisionist historian, I firmly believe this is one of the best collections of primary accounts of historical events. In addition, he turns to the writings of average citizens for his discussion, which often present a very different interpretation than the writings of those in power.