معالم السنن شرح سنن أبي داود is an educational book app developed by So Smart Apps for Android devices. The app provides a comprehensive search feature that allows users to search for any content within the book, including individual subsections and chapters. Users can adjust the font size, color, and style, choosing from eight different Arabic fonts. The app also offers various color schemes and backgrounds, including the option to set custom images as backgrounds. The app's menu includes a list of main sections, chapters, and a favorites section for quick access. Users can also take notes and save them for each chapter. Additionally, the app includes a timer for automatic scrolling and the ability to adjust line spacing for better readability. Users can copy and share any part of the book, and the app supports ten different languages.