I don't read a ton of Robert Heinlein, but every time I do, I'm stunned anew. I've enjoyed every book of his that I've read, but each one feels totally different from the rest. Perhaps not so much in style - he's a good writer who enjoys using first-person narration - as in moral. It amazes me that the same mind that penned Stranger in a Strange Land, with its Summer of Love-inspired message of peace, love, and understanding, was penned by the same hand that wrote Starship Troopers, with its cheerful promotion of fascism. Those two books are Heinlein's most famous; I've also dipped into Job: A Comedy of Justice, and came back up gasping for breath. A religious parody with multiple dimensions? How exactly did that fit into the canon?
The Door into Summer is a simply wonderful book. First of all, I think it should be read by all engineers who enjoy books. The main character in the book is an engineer, and I just loved his perspective on his career. The book does a better job of defining how engineers work, what motivates them, and how they differ from other professionals (such as scientists or technicians) than anything else I've read. One of my favorite lines from the book is when the narrator defines engineering as "the art of the possible." I think that's perfect - first of all, acknowledging that what we do IS an art, essentially creative, as opposed to a deterministic discipline such as mathematics. And it's the "possible" that gets us excited. Engineers are far more passionate about designing and building real things, figuring out how to make them work with the tools available to them; on the other hand, scientists are eager to define and build new tools, expanding the body of available knowledge without necessarily caring very much about if and how people will find practical application for their work. (Side note: it would be a lot of fun to read this book in conjunction with Vonnegut's "Cat's Cradle," which tackles the question from the scientist's perspective, and includes a similarly inspiring argument for the importance of pure research.)
That said, engineering isn't the main point of the book, just the primary perspective of the protagonist. The book is pure science-fiction. It is extremely well structured and interesting, with a cool structure that provides a lot of depth and complexity even at under 200 pages of length.
The book is set in 1970, and the main character "time-travels" forward into 2000. However, the book was written in the 1950's, so even 1970 is really the "future" as far as Heinlein's readers were concerned. It's a lot of fun to read Heinlein's predictions for these two years. By 1970, Communism had collapsed (right call, but 20 years early), the US has abandoned the gold standard (which I thought we had already done by the 50's), and science has progressed. The main technology that the book deals with is The Deep Sleep, which today we call cryogenics. For a significant sum of money, a person can be placed into a very cold state for a long period of time; they can wake up 10, 30, 100, or more years in the future, at the same age and health as they were before. The main motivation is for people with terminal illnesses who hope that cures will be discovered later on, but others also take The Deep Sleep for their own reasons. The hero decides to take it himself after he is betrayed by his partner and his lover, who steal away the business that he created.
By the year 2000, Heinlein is predicting ATM cards, although he doesn't envision credit cards. He thought that we'd have licked the common cold by then - I wish! He imagines a fabric he calls Sticktite, which I picture as being like latex. He invents what is essentially AutoCAD.
Besides The Big Sleep, the invention the book is most concerned with is robotic intelligence... basically an android. In 1970, they have invented Hired Girl, who is essentially a Roomba. Following on this success, robots are developed that can perform a wider variety of tasks, including assisting hospital patients, answering phones, washing dishes, and other fairly menial tasks.
Anyways, back to the story: I had realized early on that The Door into Summer was a time travel story, but it surprised me by REALLY becoming a time travel story. Besides the simple, kind-of-cheating forward-only cryogenic type of time travel, he also discovers a forward-or-back, instantaneous displacement form of time travel. As sci-fi goes, this one has a decent explanation... not quite as cool or intricate as in Primer, but vastly better than any Hollywood movie has done. It is described as a temporal equivalent to Newton's second law. If you have two objects of equal mass at the same location, and enough energy, you can move one forward in time by a certain amount, which will also send the other object backward in time by the same amount. It's a cool idea, and also one that leads to neat variations on the standard sci-fi concerns of causality and paradox.
I was enjoying the book all the way through, but fell in love with it during the last 30 or so pages. All the little hints that Heinlein had dropped throughout the book started to shift into focus, and I realized that nothing had happened by accident. Once the possibility of time travel had been added, I could anticipate how everything would turn out, and was happy to be proven right. The book ends on a perfectly satisfying note, both from emotionally and from a storytelling perspective.
Heinlein continues to surprise and please me. TDIS is a surprisingly touching book, and also extremely clever. It's also a neat little artifact from the 1950's: it's filled with the seemingly boundless optimism that we associate with that era. The protagonist passionately describes how everything is getting better, how the future is better than the past, and how he can't wait to see what's in store next. I feel a little sad that we've lost that kind of relentless positive attitude, but it's fun to see it captured here in its glorious moment.