This is a classic in science fiction, but not a super well known book due to, as I understand it, estate issues and only one or two editions. It is, however, the number one recommended book in my entire family, because it has an impact on how you think about the power of the mind, and what it means to travel and explore your own limitations. It's the number one book I look for in used book stores, because I've given away my last copy so many times I can never remember if I have a copy for myself to re-read, and I never seem to get it back from others.
The magic of this book is not just in the time travel which is so very well done, but also in the psychological growth that all of the characters go through from the beginning of the book to the end, and how it parallels the actual story line, regardless of where the time travel is occurring depending on the individual's time lines, (because, like any good time travel, everyone's chronological time and personal time varies, so characters are meeting at different times. This is important, and plays a huge part when you go back and re-read it the second or third time, and begin to see the underlayment of the story.)
To be honest, I never actually think of this book as being science fiction, I think of it as being a book about personal growth, the difference between what our past dictates we become, (on both a personal and a world-view level,) and how much our actions actually do matter, (there's that time travel again.)
That being said, don't bother with McQuay's other books. Seriously.