For Spanish, I use the General English-Spanish dictionary for iOS from https://www.wordmagicsoft.com (they also have Android and desktop apps, apparently). It's fantastic: it lets you look up conjugated verbs and has full conjugation tables as well. I can generally read a fair amount without consulting the dictionary (I'm working my way through Hija de la fortuna by Isabel Allende right now), but every so often I need it, especially when I get into some area of unusual subject matter. Last month I read Federico García Lorca's Romancero Gitano which really gave the dictionary a workout. His use of language is so inventive I found myself looking up words I knew because I would be thinking "does that make sense?" an awful lot. I may have to read García Lorca annually just because it expanded my mind so much.
If you are at the point of reading a book (upper intermediate) you chould probably be using a normal Spanish dictionary with the definitions in Spanish.
True, it's largely laziness that keeps me using it. And there are occasional brain freezes where this dictionary really helps (the one that I remember most clearly was when I was reading a B. Traven story in Spanish and was stumped by alitas (this was back when I still used a paper dictionary while reading). Obviously it's the diminutive of alas, but I just couldn't see it. Fortunately, I was on vacation in Mexico at the time and the tour guide was able to point out my dumb problem. My wife who is fluently bilingual (she's a native Spanish speaker and speaks English without any trace of an accent and has a book credit writing in English) (and actually, she's trilingual, she also speaks French. Makes me feel pretty stupid beside her) uses the same dictionary although much less than I do, for the occasional gaps in her vocabulary (which is often broader than my own, see http://www.dahosek.com/susurrus/ )
