Here's how the comparison works:
* Paris is presented as a desirable "volume": Lady Capulet describes him as "a handsome book" and says "he's a gentleman of excellent breeding". She implies that Paris is a good catch, like a beautifully bound book filled with desirable qualities.
* Juliet is urged to "read" him: She encourages Juliet to "read o'er the volume of young Paris". This means that Juliet should get to know Paris better and consider his merits.
* Marriage is the "completion" of the reading: Lady Capulet assumes that once Juliet "reads" Paris, she will find him appealing and agree to marry him. This is like finishing a book and finding it satisfying.
However, Juliet is not interested in Paris. She finds him bland and uninteresting, unlike the "volume" of her love for Romeo.
The book analogy is a clever way for Lady Capulet to present marriage to Paris as a desirable and natural progression, while subtly dismissing Juliet's own desires and feelings.