The US exams involve a lot of memorization; for the Technician and General exams, I highly recommend running through the questions in the $3.99 HamStudy app/completely free hamstudy.org website. For the Amateur Extra, I crammed using "Pass Your Amateur Radio Extra Class Test - The Easy Way!" by Craig Buck, and the title is completely correct: it isn't "Deeply Understand the Material on the Extra Class Test," and is useless for anything other than passing the current US Amateur Extra Class exam. "Amateur Radio Extra Class Licensing" by Stephen Horan did a bit more education and has been worth looking at since passing the exam.

I passed my exams, and while I paid close attention to all of the safety/legal things (and looked them up for Germany, where I live and later converted my license), I left fully understanding the technical parts for after I got licensed and was able to start doing. For example, I memorized a bunch of stuff about antennas to get the questions right on the exams; over the past few months, I've been modifying and scavenging the ones I bought early on and making my own.