1) do you ever wonder what inspires some of the ideas behind various novels that are in publication?
That's a hard question to answer well without examples. Some ideas are inspired by a person's personal experiences, another's vision and philosophy, another by the idea to write something people can read while laying on the beach (that is, for pleasure), but ultimately all books are written because the author wants to tell a story. Human beings like narratives. All good non-fiction books (even philosophy and history) has a structure reminiscent of a fictional work's (complete with a "pay-off" at the end).
2) Have you ever wondered about when some novels that are put out about the same time contain many similar themes
You can always tell in which decade a movie was made because, even if the story is set in a different time-period, the make-up and costumes and hair subtly mirror the present tastes. In a similar way the writing of a certain time can reflect the "zeitgeist" of that age, wether the author wants to or not. Authors tend to draw on their surroundings and sometimes they soak up the atmosphere of the time they wrote it in. Sometimes they do it on purpose too.
3) (this one applies to fellow writers too) do you ever wonder that something you've read (or written) could one day happen in real life?
Oooh, now you've opened Pandora's Box!!
There are two things that can make what you've read happen. One is when something you wrote happened a few years (or even a few centuries) later. Then there's the one when something fictional becomes true, which is different and which I'll tackle last.
Sometimes when a person write a book about something bad that is going to happen; that is for instance, he writes about a time when the United States comes under attack by terrorists becomes of various happenings and factors, and it happens a few years later, then the reason it happens is just because maybe he saw it as a logical thing to happen, even if he may doubted that it might when he wrote it. Although not entirely similar, a novel by Thomas Harris (the guy who created Hannibal Lector) called
Black Sunday, for instance, had many eerie similarities with Timothy McVeigh's terrorist act (both McVeigh and the fictional terrorist were war veterans who turned on their own country, both were indifferent to civilians, etc). There are many BIG differences as well, but those things were just setting and plot devices. It's the small things that concur. So on a certain level these things can become true, but this is rare.
The second one is a tad more chilling: someone publishes, say, a normal work of fiction, like Bulwer-Lytton, a half-forgotten 19th century author who coined the phrase "It was a dark and stormy night". He wrote a book an early science fiction book called
Vril: The Power of the Coming Race which told the story of a man who discovered a race of angel-like beings who live beneath the crust of the earth who were descendants of Atlantis. They use an ancient mystic force named Vril that is more powerful than anything modern humans can create. Despite being presented as fiction, many people of the 1870s believed the book to be based on fact (no, I'm not making this up). People (specifically occultists) began to take it seriously; many Victorians were interested in séances and contacting spirits and somehow many took the book as Gospel. Even some books published by the Theosophical Society spoke about Vril-power and Atlantis and even today people believe in occult writings inspired by the book (this is refered to as hypertextuality, I think). So in a sense Bulwer-Lytton's work became to be regarded as true, despite the fact that he never actually claimed it to be. It just hit the right nerves with many, much like
The Da Vinci Code has today.
Sometimes a person can write a satire that is meant to be both provocative and funny, only to be found that the people he satirizes are taking it seriously. For instance, an American author from the late 1800s (whose name I've forgotten) once wrote a poem satirizing American's racist perceptions of Chinese Immigrants. Later he was horrified to find that his poem was hijacked by racists who used it to enforce those stereotypes.
Bulwer-Lytton and the author whose name I've forgotten, however, did not try to mislead people, but sometimes there are bogus works that try to mislead or stir-up people on purpose, and the prime example of this is the
The Protocols of Zion's Elders, which is actually the origin of both the anti-semetic "myth" about Jews trying to take over the world (you must have heard some dumb variation on this at some point in your life) and conspiracy literature in general. It was written in the early 20th century by a man who worked for the Okhrana, the secret police of the Russian Czars, who wanted to stir up hate against the Jews. He borrowed liberally from many fictional sources which had nothing to do with anti-semetism. The largest section was copied from a short book callled "
A Dialogue in Hell Between Machiavelli and Montesquieu", but it also borrowed ideas from several other fictional sources, including a forgotten Alexandre Dumas novel about a conspiracy to overthrow the French Monarchy. The book appeared somewhere around the early 1900s and is still being used today, despite the fact that it has been continually exposed as a hoax in the 1920s. The Nazis used it, Al Queda uses it, the current Iran government uses it and even some sincere but very wrong-headed Christians use it because they think it's telling the truth (I hated typing that, but it's true). It was a malicious hoax intended for racist purposes and it's still doing it's job. So in a sense
The Protocols became true for many only because they wished to believe it for whatever reason they have (maybe because sometimes it's good to have someone to blame for your own mistakes)