The book is correct. There is no equivalent to "the" in Classical Latin.
In Vulgar Latin, the demonstrative ille (which means "that" in Classical Latin) got bleached into a definite article, with a meaning similar to English "the". That's where forms like Spanish el, Italian il, French le, and so on come from. But that wasn't good Classical style.
There is no article in Latin. You just don't translate it. "The girl is the servant of the lady" = "puella est ancilla matronae".
However, there are some cases where an article may be translated with various words: this happens when you're not really using "the" or "a" as an article, but to mean "the famous", "that famous", "a certain". For instance: "the ...
That will depend on a number of different factors. Latin is a heavily inflected language; we do this in English (think he, his, him, etc), but not to the degree that Latin does. Like in English, the ending is telling you about the word and how it's being used in a sentence.
Let's look at Latina in LLPSI. The -a ending tells you that it's an adjective that ...
The other answers already covered the most important part of your question but since you did mention
you do know those are just archaic forms of "one" and the way to express that is unus, una, unum, right? Similar to Chinese, though, Classical Latin can express this idea but usually just doesn't bother to. Use unus &c. in places ...
There is one Romance language, namely Sardinian, in which it was not the demonstrative "ille" to get bleached into the definite article but where the definite article derives from the Latin word ipse (the same), and so it became su (masc.), sa (fem.)
I'd keep my advice simple on this:
Do anything that you like.
I mean it; the rest is just a commentary on this.
If you enjoy doing something and can think of a way to combine it with Latin, go ahead and do it.
You can create or recreate — if drawing your own comic in Latin sounds like too big a project, take a comic and write new texts in Latin.
If it ...
You can use "is, ea, id." It's used sort of like an adjective; its a softer demonstrative and is often used to mean "he, her, it," on its own. But you could say "is frater" to mean "THE brother," although the common translation is "this/that brother."
Maybe you could use Spatium Sidereale? Spaceships seem to be Naves Sidereales, so it would make sense. It basically means "Starry Space". In Portuguese we use the expression Espaço Sideral so there is a precedent in romance languages.
The Morgan And Silva Furman University Lexicon states this:
space inane infinitum (Eg. D.L. 46), inane spatium cosmicum (Eg....
I think that the prefix sub-, as you yourself have mentioned, fits the bill quite well. Lewis and Short list subabsurdus, subagrestis, subalbus, subaccusare, subirascor, etc., to which I could add subfuscus, a favorite of Patrick O'Brian.