Chegg – "a company … that specializes in online textbook rentals (both in physical and digital formats), homework help, online tutoring, scholarships and internship matching.” (Wikipedia) In terms of pricing, Chegg may be a better option than Amazon. Their buy and rent fees are much lower and for additional fees you can request a tutor and help with solution problems. The app is a gateway to the Chegg world. Available at iTunes and Google Play.
WolframAlpha – is the “Wikipedia” of computational data. WolframAlpha covers mathematics, statistical & data analysis, physics, chemistry, materials, engineering, astronomy, earth science, life sciences, computational sciences, units & measures, weather, health & medicine, transportation and much more; all in the palm of your hand. Wolfram also provides separate apps for disciplines eg Calculus Course Assistant, General Chemistry, Astronomy and others. There is nominal fee. Available at iTunes and Google Play.
AutoCAD 360 from Autodesk - AutoCAD 360 is a “DWG viewing application, with easy-to-use drawing and drafting tools that allow you to view, create, edit, and share AutoCAD drawings across web and mobile devices – anytime, anywhere.” (Google Play). Free. Available at iTunes and Google Play.
Other resources may be found at 20 Apple apps for engineers (EDN), 10 iPad Apps for Engineers (ASME), 9 Android Apps for Engineers (ASME).