A Shortcut to Paradise is rich in typical expressions of the different Spanish regions and of the different social background of the characters. My copy is a translation from the original in Catalan to Castilian, the only advantage being that it was done by the author herself who is also a professional translator.
As far as I know, Teresa Solana writes in Catalan and she has translated her first two books only to Spanish-Castilian, while Domingo Villar writes simultaneously in Galician and Spanish-Castilian and his books have been published simultaneously in both languages so far.
Castilian is the official language of the Spanish state, as established in the third article of the Constitution. The same constitution, approved in 1978, also recognises other co-official languages: Basque, Galician and Catalan / Valencian. In addition to these, various dialects are used in other parts of the country, such as Aranese, a dialectal variety of the Gascon language which, in the Arán valley, is considered a co-official language with Catalan, according to the Catalan Statute of Autonomy; Aragonese, spoken in areas of the Pyrenees in the north of Aragon; Leonese, used in the provinces of León and Zamora; Asturian or ‘bable’, in Asturias and neighbouring areas. There are also various dialectal varieties associated with Castilian, such as Andalusian, Canarian, Murcian and Extremaduran. Equally, other minority languages are also used in Spain, such as Caló (a variant of Romany spoken by the gypsy community) and Arabic dialects (essentially spoken in the autonomous cities of Ceuta and Melilla).
Above information was taken from A multilingual country.