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Are there any good/decent books about Mozarabic in Spanish or English?

Nessuno?

No, there aren't.

There are three books quoted on the Spanish wikipedia page on Mozárabe that may fit your bill:
Galmés de Fuentes, Álvaro (1983). Dialectología Mozárabe. Madrid: Editorial Gredos.
Peñarroja Torrejón, Leopoldo (1990). El mozárabe de Valencia : nuevas cuestiones de fonología mozárabe. Madrid : Editorial Gredos.
Sola-Solé, Josep M. (Josep María) (1973). Corpus de poesía mozárabe; las harǧa-s andalusies. [Barcelona] Ediciones Hispam.

A review of the first book can be found here.

I have no idea how decent these books are or whether they can be found in a library near you, but it indeed doesn't seem like there is a lot available.

hwhatting wrote:Peñarroja Torrejón, Leopoldo (1990). El mozárabe de Valencia : nuevas cuestiones de fonología mozárabe. Madrid : Editorial Gredos.

I consulted this one when I was working on a Mozarabesque conlang. I found not all that helpful.

hwhatting wrote: A review of the first book can be found here.

Mistake alert - the review is actually of the second book ("El mozárabe de Valencia") . Anyway, linguoboy has actually used it, so as he didn't find it useful, it's probably not worth the bother to chase it up.

The Spanish Arabist Federico Corriente has many books on the subject, most of them focused on Hispanic Arabic and its influence about the Romance languages spoken in the Peninsula (results from a Google search):

- A grammatical sketch of the Spanish Arabic dialect bundle (1977).
- Las lenguas prevalencianas (1986).
- Arabe andalusí y lenguas romances (1992).
- A dictionary of Andalusi Arabic (1997).
- Diccionario de arabismos y voces afines en iberorromance (2003).
- Dictionary of Arabic and allien loanwords (2008).

But his book "Romania arabica: tres cuestiones básicas: arabismos, mozárabe y jarchas" (2008) deals more specifically with Mozarabic. The only drawback is it's quite expensive (around 35€) for a tiny book.