Mark Asch, Maëlle Nodet and I have written a textbook on data assimilation. After three years of preparation and efforts, it has been published by the Society of Industrial and Applied Mathematics (SIAM). It is meant for beginners with a broad introduction to the principles and fundamental methods of data assimilation in the first part. In a second part, it contains more advanced topics such as current status of nudging, reduced-order data assimilation methods, the ensemble Kalman filter, and a review of the ensemble variational methods. A third part collects lots of short examples in the many fields where data assimilation is used. Check it out .