Pretty much every book out there which claims to teach the readers to appreciate classical music, has significant amount of theory in it. Ex: What to listen for in Music by Aaron Copland. Is it not possible to enjoy a classical piece without having any theoretical knowledge? If it is possible, kindly point to correct resources which helps in achieving the goal.

Of course it is. And most people do.

And, while any piece more extended than a simple song probably does involve a 'journey' of some kind, there's no need to invent a storyline.

Is it possible to enjoy driving a car without understanding how the gearbox works? Watching t.v. without understanding the electronics involved?

Yes, of course! In fact, once theory is studied and understood, most music is listened to and appreciated in other ways. Not better or worse, just different.

Think about it: if music was only appreciated if the listener was versed in theory, it would have probably died the death well before now.

A lot of my students have commented that they don't listen to music in the same way, once they understand the tricks used and the theory involved. Which can be detrimental, as they then tend to analyse it more, delving deeper into it, to different layers. But that's not necessarily good or bad as far as listening and appreciating goes, just different.

This may get closed as 'how does a layman understand a piece' is very subjective. Take ten people who've listened to a piece of music, and there will likely be ten different reactions, interpretations, or whatever you want to call it, that the music will have given them. O.k., a minor piece may(?) give a sad feeling , but go deeper, and it could be interpreted as nostalgic, sorrowful, serious, tear-jerking, or lots of other emotional effects. Then you'll get the ones who liked the Tierce de Picardie at the end, or the way it modulated in the middle, or the way the staccato part came over. Get the idea?

Yes, it is. They might probably say this is a opinion based question. But everyone who has ever heard some classical music as a baby will confirm that this music is in his mind and his soul like he drunk the milk of his mothers breast.

The adaption to classical music is like the learning of a language, if you've learnt it as a child you will understand it without grammar and theory. But later it will be more difficult like learning a foreign language than the mother language. Theory could be a good help to make it easier but more important are the elements of what you got when you was brought up.

Is it possible to fully enjoy a classical music piece without knowing
the theory behind it? If one should know music theory to enjoy a
piece, was/is classical music for aristocrats? How would layman
understand a piece?

So to answer your question: it is not only possible, it would be quite ordinary that one could understands and enjoy classical music - if he has been adapted to it or if he really wants to listen to it.

Enjoy? Yes, because otherwise almost nobody would ever like classical music enough to study all that theory and composition.

The first record I ever bought, when seven years old, was Holst's "The Planets". I'd heard "Mars", I can't remember where, and was just stunned that such a thing could be. It grabbed me deep down in my brain, insisted I listen to it again and again, and simply wouldn't let go. In some ways, it never really did.

Now I play trombone, I've won scholarships and studied with great players, I've done a fair amount of conducting and arranging, and I've played "The Planets" in concert many times. I understand how Mars is put together as a piece of music in ways I didn't even suspect existed when I was seven.

Last week I bought a new CD, by the Szeged Trombone Ensemble, which includes their arrangement of "Mars". I didn't know it was on there, it just came up in the playlist when I first put the disc on; and instantly, there I was, spellbound again. Even as my conscious mind hummed away, breaking down the chord structure and that fantastic ostinato, the bit of me that was there when I was seven and hasn't gone yet just stared at this gigantic musical thing lumbering past, spellbound and awestruck.

I can't say that I understood it in any intellectual way when I was seven. But could I enjoy it? Yes. Yes, I could.

Short answer:

Yes

A bit more detailed answer:

Classical music does have some defined rules about them, but that is more as a recepie to why the music is as enjoyable. The classical composers did know about some of these rules, but most rules were added as recognised patterns to the music after the fact because the composers just tried out stuff that sounded good. They sounded good because they follow some rules of nature that people instinctively recognize as pleasant to listen to. Some of these rules can even be recognized in birds singing patterns.

Sometimes the composers made things that deliberately went against the rules, just to make the music more interresting, because if everything follows the rules all the time, the music would be too predictable, and thus boring to listen to (even if you do not know what these rules are).

...every book out there which claims to teach the readers to appreciate classical music...

There's the 'problem'. No one needs to be taught to enjoy music. Well... excluding 12-tone, aleatory, and other avant garde music. (Only half-joking.)

Basically when an artist does not regard their audience with contempt there is no need to educate the audience, because they will feel whatever the art expresses as a natural cultural expression.