TITLE: A book you would like to write
QUESTION [67 upvotes]: Writing a book from the beginning to the end is (so I heard) a very hard process. Planning a book is easier. This question is dual in a sense to the question "Books you would like to read (if somebody would just write them)". It is about a book that you feel you would like to write (if you just have the time). A book that will describe a topic not yet properly discribed or give a new angle to a subject that you can contribute.
The question is meant to refer to realistic or semi-realistic projects (related to mathematics). Answers about book projects based on existing survey articles or lecture notes can be especially useful.
Of course, If you had some progress in writing a book mentioned here please please update your answer!

REPLY [3 votes]: I would like to write 'the best book on differential geometry', a genuinely pedagogical, useful and visual introduction to differential geometry for students who are coming to the subject for the first time with no prior knowledge.
None of the existing textbooks really meet these criteria in my opinion.
Edit: There is a new book out by Tristham Needham called ''Visual Differential Geometry'' which claims to meet my criteria above, hopefully this lives up to expectations.