The eBooks o eBooks have become the successors of the well-known PDF, being replaced by the format .epub (usually), which allow comfortable digital reading from various devices and are advantageous to adapt to the size of the screens.
EBook Readers there is something for all tastes, personally I have tried several and in my humble opinion the best of all -so far- is FBReader.

Some features to highlight from FBReader:
- Multiplatform, compatible with Windows-Linux-Mac OS X-Android-Blackberry, etc.
- Multilanguage, available in Spanish.
- Lightweight installer file, 5 MB (on Windows).
- Comfortable, customizable, easy-to-read interface, letter-color-rotation.
- Very low resource consumption
- Search for eBooks on the Web.
- Internal word search engine.
- Library Organization.
- LPG
If what you need is a good eBook reader, fast and without complicated settings or designs, you know that FBReader it is one of the best options to consider.
Official site: FBReader