I generally review books in the language I read them, but this is specially for English readers. Lots of English reviewers here seem to think this is just a swashbuckling adventure novel, and the plot is too straightforward for that.
But it's not. It is a period piece, a reconstruction of atmosphere, language, and character of Spain in that - really, quite famous - time. So all the ramblings and poems and descriptions serve not the plot, but atmosphere, and here you are. The plot, being so simple, seems quite realistic, though not wildly exciting. As for poems and style, they might have lost something in translation. But I think the substance is enough to try to understand the time and the people.
And by the way, lots of other books by Pérez-Reverte are a great deal darker, bordering on macabre (I've read four so far).