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A kōan is nothing more than a puzzle that seems self-contradictory or paradoxical, and yet contains a teaching of truth in it. Those who want a book about it are probably unqualified to create kōans to begin with.
That being said, each sangha (whether online or not) can adopt kōans as they see fit, whether validated by a group of masters or not. Such validation serves only to give a thorough review of the proposed kōan, and doesn't pertain to whether or not it validly teaches, since the acceptable answers can vary greatly between students and even teachers. The abstract quality of a kōan to begin with makes it an uncertain thing at best, anyway.
There is not any particular qualification, other than understanding what a kōan actually is, in its full and ineffable truth. Any book on the subject would necessarily have to ignore the ineffable (e.g., indefinable) part of that, and so would completely miss the point of the abstract. But if it was to teach you what you need to get to the level of abstract comprehension, that would suffice, and there are lots of those.
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You know the definition of "kōan" is "case," right? As in "law case" for instance; or as in "the case of the purple pimpernel." Maybe because Zen did not have the large body of writings that Indian Buddhism had, examples were used as a teaching method. After a while the best - maybe most profound - examples were collected, as in the Blue Cliff Record. There was never a secret to constructing them, they are each a record of an instance that happened.
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I don't know of any book on the history and development of koans, but I'm sure one exists if you care to search. Many collections have some background information in the introduction, and the individual commentaries often include background information. Further discussion of koans is scattered throughout Zen texts in general.
A koan is a koan because it makes an important point about practice and has been used for teaching for a long time, so it's debateable whether or not it's possible to create new ones. There's over a thousand traditional koans in existence, so there's no shortage :)
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This book might be useful: The Koan. Texts and Contexts in Zen Buddhism. By Steven Heine e.a. With contributions from several specialists in the field