This is the 1280th most frequent German word.
erstes
"Erstes" means "first" in English.
In this sentence, 'erstes' is used as 'ersten', the genitive form referring to 'Tag' (day); it designates sequence or order as 'first.'
Am ersten Tag der Schule war ich sehr aufgeregt.
On the first day of school, I was very excited.
Here, 'erstes' appears in the phrase 'zum ersten Mal,' indicating an ordinal aspect, specifying the first instance of an event.
Er hat zum ersten Mal den Marathon gelaufen.
He ran the marathon for the first time.
In this context, 'erstes' describes 'mein Buch,' attributing a sequence or precedence, implying it is the initial book written by the person.