I had a tough time getting into this book. It's written entirely as progress reports (more like journal entries) by the main character, Charlie. And at the start of the book, Charlie is barely literate. For me, that made the first 40 pages or so a struggle to get through. But then Charlie undergoes a treatment that is never fully defined (there's some sort of surgery involved) and his intelligence grows. And luckily, the book becomes much easier to read as Charlie learns standard spelling and grammar.
Keyes used that premise, a character progressing from mental disability to extreme genius, very thoughtfully. He writes about how both a lack of education and the extremes of education can become isolating. Before the surgery, Charlie was often the butt of jokes by people he had thought were his friends, who were willing to take advantage of his lack of understanding to give themselves an ego boost. Afterwards, those same people become intimidated by his rapidly increasing intelligence, because his new intelligence made them feel inferior to him. At the same time, Keyes doesn't give Charlie a pass, showing how he allows his increased intelligence to divide him from people around him who are, in his opinion, less educated. And then there's also the idea, showing up again and again in the story, that pre-surgery Charlie was a person, every bit as much as post-surgery Charlie, and that he had deserved to be treated with respect.
Of course, this book is also a product of its time. It's fitting, because Flowers for Algernon was being written as taking place in the present day. There are things which are jarring to modern readers that would not have been so when Keyes wrote this book in the late 50s. Apparently, it was acceptable to call people with mental impairments morons, something that just wouldn't pass today. I could get used to it being on the page, but I don't doubt that many readers would have trouble with that. I think I was able to accept that this had been standard and inoffensive at the time because Keyes was, overall, writing thoughtfully.
This is a book with an emotional impact that leaves the reader with much to think about. At times, it can be slow. We are essentially following Charlie's thought process through his progress reports, and he isn't always hurried. But in the end, that turns out to be a good thing. We get to know Charlie much better than we might have otherwise, so we become more invested in what will become of him. And it gives Charlie's last progress report an additional weight that it may not have had otherwise.