This is the 339th most frequent Spanish word.
ningún
"None" or "not any."
'Ningún' is applied here in a negative statement, preceding 'perro', to signify an absence of something.
No vi ningún perro en el parque.
I didn’t see any dog in the park.
The use of 'ningún' emphasizes that no singular item (book in this case) met the criteria.
Ella no encontró ningún libro que le interesara.
She didn’t find any book that interested her.
In this sentence, 'ningún' is used before 'problema', a singular masculine noun, to mean 'not any' or 'no' in English.