If you are serious about translation, you should think about several dictionaries. I have many in my library but just on the top shelf above my computer I can count 21 of them, plus books on grammar, style, phrasal verbs, etc.
A good bilingual general dictionary is the Diccionario Oxford Esencial. I also like the VOX New College Spanish and English Dictionary. As for monolingual dictionaries, I recommend: Diccionario de uso del español, by María Moliner, Diccionario esencial de la lengua española, published by the RAE, Diccionario panhispánico de dudas, published by the RAE, and Diccionario de dudas y dificultades de la lengua española, by Manuel Seco.
Then you should think about your specialization dictionaries, depending on the subjects on which you specialize, whether medical, business, legal, marketing, healthcare, technical, engineering, etc., etc.
I hope I was of help.
