This app was mentioned in 1 comment, with an average of 3.00 upvotes
There are tons of apps for specific books, so it depends what you want.
General:
BetaMidrash (android only)
Genre specific:
Yolaroo (tanach and Mishnah only)
real clear daf (daf yomi)
Chok L'Yisrael (various apps - just search in English and Hebrew)
Book specific (just search in stores):
ילקוט יוסף
רמבם פלוס (mishnah torah from mifal mishnah Torah - probably the best edition out there).
עולת ראיה
If there's another specific book you want, try searching for it (especially in Hebrew) - if it's common, there's a good chance someone has made it into an app.