This is the 8069th most frequent Polish word.
niezłą
"Pretty good" or "quite good."
The word 'niezła' is used here as an adjective to describe the book, indicating it was good or noteworthy, commonly conveying a positive evaluation.
To była niezła książka.
It was a pretty good book.
In this context, 'niezłą' emphasizes the level of the adventure, indicating it was remarkable or amusing in some way.
Miałem niezłą przygodę wczoraj.
I had quite an adventure yesterday.
Here, 'niezły' refers to the day, indicating it was pleasant or satisfactory for an activity like going to the beach.