The meaning "between the lines" is referred to as the 'subtext'.

If someone says something to you in a straighforward manner the meaning is usually quite clear isn't it. But what if they wink at you as they're saying it ...? Then that suggests that they're implying something else ... and that is the 'subtext' to what they said ... that is the 'meaning between the lines'.

Most products of thoughtful creative endeavour incorporate a subtext, which is a meaning that isn't explicitly stated, but is nevertheless implied. Discerning what the subtext might be is a skill one acquires as the Mind matures, and develops the ability to see things below the surface and in shades of grey, rather than in simplistic black and white terms.

Subtexts exist in paintings, the written word, music, and movies, and are frequently used by advertisers to seduce you into buying their products (where subtexts are called 'subliminal messages', being meanings that you pick up subconsciously, below the level of language).

To take one example: On the surface, the movie "The Truman Show" is just an entertaining comedy about a guy whose entire life is played out on TV without him knowing about it. So, is that all it is ...? Not really, for there are many many subtexts present ...

... it's also about the risks of allowing a mediated reality to replace actual Reality. Or it explores what could happen if Reality TV was taken to it's ultimate conclusion. Or it's about how individuals have a right to control their own destiny and not have it scripted by someone else!

And at a still deeper level it looks at the question about whether it's likely that God has a plan for our lives down to the very last detail (like the director had for Truman), or are we in fact entitled to absolute freedom of will, rather than be God's little puppets having our strings jerked all the time according to some plan from 'on high' - as was happening in Truman's life.

These are all subtexts to "The Truman Show", and there are many more besides, and untangling them all and critiquing them can be a very rewarding, fascinating, and educational experience!

OK then,
All The Best!

There maybe more than one meaning from the author of the written words.

Obviously, all this is speculation on my part as I was not on the committee to invent language thousands of years ago, but here's my thoughts.

Language was created, one way or another, as a means to transfer information and experience from one entity to another. It is also a way to control the world, to restrict it. By engraining in every child what a "book" is, what "liberty" means, what is "right" and "wrong" we enforce our beliefs onto another. Luckily, language is very flexible and we are often intelligent enough to find ways to transcend the limits of one phrase by incorporating others. We can explain why what may be acceptable in one society is taboo in another, why a single physical book and contain multiple books within it...these are poor examples, but I think, hope they are enough.

We read between the lines, we use context, tone, body language, assumptions, prior experience and much more because the ideas that one person is trying to express to another person are not the same thing as the words used. A map of a territory is not the same thing as the territory, no matter how accurate. Subtleties will be different or missing. No map by itself is sufficient to totally explain a land. No combination of words, however perfectly chosen or numerous, can ever hope to capture the totality of the idea one is attempting to express.

We also cannot help BUT read between the lines because the very processes used by the speaker to translate abstract ideas and experiences into words are not identical to the processes the hearer will use to translate those finite words (and tone, body language, etc) back into an abstract understanding. We have to draw on contexts constantly to make sense of what is said because no two people have the exact same background, the exact same understanding of any event, any word.

P.D. Ouspensky has written in..I believe the Psychology of Man's Possible Evolution how ridiculous communication can be for just these reasons.

Maybe the words are written in a way to cause us to go deeper to discern the meaning or read between the lines because by doing so we go deeper within our own being to discern what it means to each of us as an individual and by so doing we each can take our own truth from the words. And as Fred so rightly pointed out there may be more than one meaning contained therein but each of us will interpret exactly what is right for us.

I see the key in the concept of Meta-communication.

Etymologicaly, the term derives from meta (gr. meta= "after") added to root notion 'communication'. E.g. "metaphysics", suggesting 'beyond' physical world, or for expressing the changing, transformation, transcending.

Meta-communication is a form of communication that means different things at different levels, much pending on context, what and how much intends to express or to suggest the communicator and abilities of receptor. In the writ language, some of tools are the "nuance" of selected word within multiple possibilities serving the same meaning, punctuation marks, topics of sentences, etc. For the life language, it is add, the mimics - especialy the expressions of sight, body attitude, and pantomimics, again "nuances" of voice etc.

More about metacommunication (only some sources):

Book "Communication and Metacommunication in Human Development" by Iaan Valsiner;

http://veryard.wikispaces.com/

http://www.ics.uci.edu/redmiles/chiworkshop/papers/Yetim.pdf

http://en.wikipedia.org/wiki/Meta-communication

NLP uses intensively meta-communication, also as-named "street"-hypnosis (see Google).

It says that, along his evolution, man used communication (language) for exprime himself - initialy, but leter, more for hide himself.