This is the 4385th most frequent Russian word.
вашу
"Вашу" is the accusative or feminine form of "your" (formal or plural).
Here, 'вашу' is the accusative form of 'your' (formal or plural), describing the noun 'книгу' (book) in a direct object position.
Где я могу найти вашу книгу?
Where can I find your book?
'Вашу' in this sentence modifies 'рекомендацию' (recommendation) and is in the accusative case, reflecting the action of reading.
Я прочитал вашу рекомендацию.
I read your recommendation.
'Вашу' in this sentence would be grammatically corrected to 'ваш', though it is intended to agree with the gender and case of 'вопрос' (question).