A medida que avanzaba la reseña, me fui dando cuenta de que la contraportada no explicaba bien la trama.

It means as or as/while something was progressively happening.

In this sentence, a medida que shows that two things developed at the same time:

• the review kept progressing
• the speaker gradually realized something

So A medida que avanzaba la reseña... is basically As the review went on... or As the review progressed....

Avanzaba is the imperfect, which is used for an ongoing or unfolding past action.

That fits well here because the review was not seen as one finished event, but as something moving forward over time.

• avanzaba = was progressing / kept going
• avanzó = progressed / moved forward as a completed event

After a medida que, Spanish very often uses the imperfect because the idea is gradual development.

The subject is la reseña.

So this means:

• the review was progressing
• not I was progressing through the review

If you wanted to say As I got further into the review, you would more likely say something like:

• A medida que avanzaba en la reseña...
• A medida que iba avanzando en la reseña...

So the original sentence focuses on the review as it unfolded, not directly on the reader’s movement through it.

Here, reseña means review.

It is close to crítica, but there is a slight difference in feel:

• reseña often sounds more neutral or descriptive
• crítica often sounds more evaluative, more like a formal critical review

In many contexts they can overlap, but reseña is a very natural word for a book, film, or cultural review.

Because me fui dando cuenta suggests a gradual realization.

• me di cuenta = I realized / I noticed
• me fui dando cuenta = I gradually came to realize / I slowly started realizing

This uses the structure ir + gerund, which often gives the idea of something developing little by little over time.

That makes perfect sense here: while reading the review, the speaker did not understand everything at once. The realization built up gradually.

Because the verb is the idiomatic expression darse cuenta de, which means to realize or to notice.

It is a pronominal expression, so you use these forms:

• me doy cuenta
• te das cuenta
• se da cuenta
• nos damos cuenta

So you should learn it as a whole chunk:

• darse cuenta de algo
• darse cuenta de que...

It does not mean literally to give account in this sentence. It is just a fixed expression meaning to realize.

Because darse cuenta requires de.

You can have:

• darse cuenta de algo = to realize something
• darse cuenta de que... = to realize that...

So:

• me fui dando cuenta de que la contraportada...

is completely correct.

This is not a mistake. Some learners worry about de que, but here the de is required by the expression darse cuenta de.

Yes. Both are correct:

• me fui dando cuenta
• fui dándome cuenta

With verb structures like this, the pronoun can often go:

• before the conjugated verb, or
• attached to the gerund

In this sentence, me fui dando cuenta sounds very natural and neutral.

La contraportada is the back cover of a book.

In context, it often refers not only to the physical back cover, but also to the text printed there—what English might call the back-cover blurb.

So here the meaning is probably:

• the text on the back cover did not explain the plot well

Again, the imperfect fits the idea of an ongoing description or a characteristic the speaker was noticing.

• no explicaba bien la trama = it wasn’t explaining the plot well / it didn’t do a good job of explaining the plot
• no explicó bien la trama = it failed to explain the plot well, as a more completed event

The imperfect works well because the speaker is describing what they were gradually realizing about the back cover.

Here, la trama means the plot or the storyline.

That is the usual meaning when talking about a book, film, or series.

In other contexts, trama can have other meanings, but with literature or cinema it normally means plot.

Yes, it is natural.

It means that the back cover did not make the plot clear enough.

Other verbs could also be used depending on nuance, for example:

• resumir la trama = to summarize the plot
• presentar la trama = to present the plot
• plantear la trama = to set up the plot

But explicar bien la trama is perfectly normal and easy to understand.

The sentence combines several features to create a very natural sense of gradual discovery:

• A medida que = introduces parallel development
• avanzaba = ongoing background action
• me fui dando cuenta = gradual realization
• no explicaba = descriptive imperfect, not a one-off event

So the whole sentence feels like:

• the speaker kept reading
• little by little, they realized
• the back-cover text was not doing a good job of explaining the plot

That is why the sentence sounds smooth and quite literary.