Name of a sci-fi story about time travel

Question

There was a short novel I found in my library few years ago, whose title nor author I don't remember. It was in a hard sci-fi setting: there was a pair of characters (man and woman) which traveled through galaxy. The story was told in a chronological order, but because of time travel (for which a binary star system was used), they often met older or younger themselves, multiple times. There was even a diagram (done by author) of their lives and how the story actually was seen from their point of view: with events ordered as they saw them.

I remember there was a talk about seeing the world from one or many perspectives at once--it was even called with some special words, but as I read only a Polish translation, I don't know how it was named in English.

EDIT: the English words used were: Simplex, Duplex, and Multiplex (thanks to Winchell Chung for adding them in a comment). /EDIT

I think the original was in English, and that it was written before 1990... but I'm not sure. It was a single book, it had maybe 100 pages.

One of the first scenes was that boy (main character) lived on a not very interesting planet and wanted to escape from it. He knew an old lady living on that planet. That lady once showed him a girder bridge that was built on that planet--she said it was modeled after a bridge on Earth. She said that as long as he stands in one place, he cannot see that it is a girder bridge, as he sees it only from one perspective. That lady was actually the same woman he will meet later on a ship (as a young girl), but in her last stage of life.

What was that story name?

Answer 1

By the description, it sounds a lot like Empire Star by Samuel R. Delany.

As the narrative opens, we meet Comet Jo at eighteen years of age. He has spent his entire life in a "simplex" society on Rhys, a satellite of a Jovian planet orbiting Tau Ceti. (At first it might seem that "simplex" means "simple" or "unintelligent," but after Jo's encounter with the "Geodesic Survey Station" at the latest, it will be clear the notion is much more complicated.) Jo comes upon the wreckage of a spacecraft and encounters two survivors. The first is quickly dying and asks Jo to bring an important message to Empire Star moments before passing away. The other is a lifeform known as Jewel. Jewel is a tritovian in crystallized form, and in that state can easily view situations from several points of view, thus enabling narration from the point of view of the omniscient observer.

Jo quickly leaves Rhys in an attempt to deliver the message to Empire Star, and on his journey he meets several other characters along with a race of creatures known as the Lll. The Lll are incredible builders—not merely of structures, but of ecosystems, societies, and ethical systems. As such, they have been enslaved. However, in order to protect the Lll, the Empire has created a phenomenon known as “the sadness of the Lll”—any being who owns the Lll suffers from a constant, overpowering sadness. This sadness increases geometrically with each Lll owned and with how much each Lll builds, so it is only possible to own a few Lll at a time. Indeed, just being in the presence of the Lll is a heartbreaking experience for even non-owners, a lesson that Jo learns early in his travels.

The story then follows Jo over the next few months. Once he reaches a certain point in his maturity, knowledge, and ability to perceive events around him, the linear narrative stops and the reader is left with a few pages of important events not arranged in a strict order; by this point, the reader may have learned enough to sort out the tangle.