Online courses for German at levels A1, A2, B1
You may use, instead of an e-book, the free online tool offered by Deutscher Volkshochschulverband
Deutscher Volkshochschulverband is the nation-wide association of "Volkshochschule" adult education centers. You find a Volkshochschule (roughly: "folk university") in almost every city of recognizable size. There are about 900 Volkshochschulen in Germany. Usually they are owned or run by the municipality, sometimes by a Landkreis (county). In any case they provide high-quality but affordable (through public subsidies) courses in topics such as language, arts, health, everyday skills, soft skills for business, some vocational training etc. In recent years providing German classes for newcomers to Germany has become a major activity of Volkshochschulen.
www.ich-will-deutsch-lernen.de is an online plattform with lots of well-structured exercises for learning German on the levels A1, A2, and B1. You can create your personal profile and study whenever you want, continuing your activities where you interrupted them last time. The graphics of the platform is a bit old-style, unfortunately (not up-to-date in terms if internet design), but the contents of the courses are good.
Fiction books / belles lettres
If however you are searching for fiction books (novels, short stories, etc.) in relatively simple forms of German, you may have a look at what this publishing house offers:
Verlag "Spaß am Lesen"
These are fiction books written in so called Einfacher Sprache (simplified German), using a less-wide vocabulary, shorter sentences, and sometimes a somewhat simplified grammar. This may be helpful for very beginners. But these books are not for free.
Personally I would rather choose original literature, and pick something that is really interesting for me. There will be lots of unknown words in the book, but looking them up in a dictionary (and seeing them used in context) is what brings you really forward with the language in question.
It fully depends on your taste of course, but why not giving Karl May a try? He is who formed the linguistic background of generations of German native speakers who read this in their teens. Rather than the famous Winnetou I-III I would recommend, though, the less known stuff such as Das Buschgespenst, a detective story playing in a poor village in Saxonia in the 1860s with - perhaps unexpectedly - profound criticism of social conditions of the time including exploitation through capitalists and the connection between unrestricted capitalism and organised crime. Does it ring a modern bell, perhaps? (And, no, Karl May was definitely not a socialist, rather a Christianity-based moralist.)
I suppose this is available on Project Gutenberg. (Sorry, I cannot check for it, as Project Guttenberg is currently unavailable in Germany due to a legal conflict regarding access to works of Thomas Mann, Heinrich Mann and Alfred Döblin. - Background on this here)
A free version for download in epub-format is here:
Karl May: Das Buschgespenst (epub)
Note that all books by Karl May that are easily available are heavily edited language-wise. There is not much left of the linguistic peculiarities of 1860s German writing; the texts were "improved" (allegedly) to be "suitable" (allegedly) for children and teenagers, and the various editors followed a mindset of their respective times, i.e. the 1930s, the 1950s, the 1970s. It is a shame in terms of how to treat literature, but it may be good for those who want to learn contemporarian German using such books. You have a very "normalised" language (and orthography) there.