The world of smartphones and PC from my angle
The most important book required while learning a new language is a Dictionary. This dictionary should be bidirectional i.e. written in both the universal/your language and the new language. In this case Deutsch to English as well as English to Deutsch.
Dictionaries are available in two ways: physical and the digital formats. Carrying a dictionary physically to where ever you go can be quite annoying at times. Besides, there is also a problem of outdated content.
Digital dictionaries are more up-to-date and are easily accessible. Dict.cc is an awesome website which includes large databases. One can translate the Deutsch words to around 51 different languages. One big limitation of Dict.cc is its requirement of internet access but that is not for everyone.
If you are lucky enough to have an Android or iPhone/iPad/iPod touch, this is a cool app which is not only free of cost but also contains the offline translations, so no internet is no problem at all, although it does require the downloading of either the concise (~10MB) or full offline databases (~180MB) .
A team of brilliant developers by the name of Mobile Systems have come up with a variety of handy dictionaries for mobile phones. One of the work is this dictionary, which is available for nearly all mobile Operating Systems – Android, iOS, Symbian, Java etc.
The concise oxford dictionaries are another great masterpieces of the Mobile Systems and are also supported by nearly all types of mobile OSs.
This helpful dictionary application comes with audio as well so that you may understand how to pronounce certain words. It also supports a variety of different mobile OSs.
Yet another great dictionary for Android, iPhone and Symbian phones is this dictionary which like the SolvoEd dictionary, comes with audio.
Of course there are other useful dictionaries as well, I have just mentioned here the ones that I found worth sharing.