Chessbook is an iPhone app developed by Chess Book Inc. It is a free educational and reference program designed to help users build a bulletproof opening repertoire. With Chessbook, users can create a custom repertoire, train using spaced repetition, and find gaps in their repertoire automatically.

The app offers a fast and modern interface, making it easy to navigate and use. It allows users to avoid obscure moves and handle transpositions effectively. Chessbook also provides a feature to find mistakes in online games, giving users valuable feedback on their gameplay.

One of the key features of Chessbook is its ability to calculate the user's coverage per opening, ensuring they always know what areas to work on. The app also helps users pick the best moves by providing stats from master games, engine evaluation, and results at their level.

Chessbook uses spaced repetition, a scientifically proven technique, to help users memorize openings quickly and thoroughly. It allows users to collect all their openings in one place, combining openings from multiple sources to create a custom repertoire.

For those looking to improve their opening repertoire and analyze their gameplay, Chessbook is a valuable tool. It offers a range of features to enhance the learning experience and help users become better chess players.