I own a number of paper and eBook Spanish-English and Spanish-only dictionaries. The best Spanish-English dictionary is the Collins unabridged which unfortunately is not available in mobipocket. It is available online at
http://mobile-dictionary.reverso.net/english-spanish. Of the available mobipocket e-book dictionaries I´d place them in this order:
HARRAPS (Much more complete than the following which are all abridged)
Cambridge Klett Compact
Merriam-Webster´s
Pocket Oxford
If Collins or Harraps doesn´t have the word I need then I go to my Spanish-only dictionaries listed below in order of usefulness:
Diccionario de la Lengua Española 2001 (RAE)
Diccionario General de la Lengua Española (Vox)