This is the 61st most frequent Spanish word.
primer
"Primer" in Spanish means "first" in English.
Here, "primer" modifies "libro" to indicate its numerical order in a sequence.
Este es el primer libro que leí.
This is the first book I read.
"Primer" here qualifies "rastro," specifying it as the initial trace observed in the context.
Encontré el primer rastro del animal.
I found the first trace of the animal.
In this sentence, "primer" describes "paso," denoting its position as the beginning or initial step.