For instance, if I asked someone, "Ĉu vi legis ĉi tiun libron?," and they said, "Mi legas tiun libron," then that would indicate to me that they are in fact in the middle of reading the book right now (for instance, the last time they set it down they were on page 53).

This is no doubt a possible and valid interpretation. It could however also mean "I read this book (every year at least once, because I love it so much.)

Same if you ask somebody "Ĉu vi fumas?" If they reply "Jes, mi fumas." this doesn't necessarily mean that they are smoking right now, it can also mean that they smoke regularly, i.e. that they are a smoker. Context will tell; or a follow up question.

The as-form has two basic functions in Esperanto: Marking the present tense and marking gnomic tenselessness.
Present tense: Nun mi bezonas dormi.
Gnomic: Ĉiuj homoj bezonas dormi.

Most IE-languages use the present for gnomic expressions. One exception might be Ancient Greek, where they use the aorist instead. Some other languages (non-IE) have a special tense (or "untense") for gnomic expressions.

Indeed, so we can come up at least three interpretations for that "mi legas tiun libron".
 

• I am in the middle of reading the book right now i.e. holding the book in my hand.
  
• I am in the middle of reading the book, even if it right now lies on the table.
  
• I read this book every year at least once.

Note, while for the first interpretation "ĉi tiun libron" would be more accurate, there are esperantists who use the "ĉi" modifier only when it is necessary to distinguish between two or more objects/point-of-references. For instance "Movu ĉi tiun seĝon tien kaj tiun seĝon ĉi tien" (quite likely the person points with their finger which chair and whereto), but should there be only one chair to move, they would just say "Movu tiun seĝon tien". (And this can be tad confusing for speakers of language with more nuanced distance contrasts.)