Ann Coulter's Treason is a true eye opener. Like you say, it's dismissed as a screed, but it's actually a very well researched and important book. I don't think the Left hasn't managed to lay a finger on any of its facts or arguments. At this point, the Left has lost the McCarthy debate. They do still have the McCarthy meme though, which remains one of their favorite tools.
If you liked Witness, you should read David Horowitz's Politics of Bad Faith. This book is not well known but in my opinion is his best. Its most impressive chapter has his letters to and from a few of his hard-left friends around the time he was breaking with the Left. It's him explaining his betrayal, laying out the case against his old philosophy. I don't know if it's because he was writing it friends or what, but it's just incredibly powerful stuff. That book is one where I can remember where I was when I read it.
“Radical Son” was another book that was a real eye-opener for me. After reading it, I understand the hate the left has for him, because he is considered to be an apostate of the liberal religion.
He was red to the core until he worked with the Black Panthers, and they murdered a woman who worked with them. That opened his eyes.