As the HP41 repair book has already passed the 500 units sold mark, I have been challenged to write a book on the HP15c.
Many years ago, there was a book called "An Easy Course in Programming the Hp-11C and Hp-15C (Easy Course Series)", by Ted Wadman and Chris Coffin, which I have not been able to get hold of. Then you have the fantastic user manual and also the Advanced functions manual - in itself a computer science crash course. Then there are many sites that have several programs, but nothing like a common respository with all known applications.
I am not the only one to love the HP15c. Its compact form factor, the ability to use it with both hands for a faster keying in, the complete and consistent fucntion set, the good memory sze for its time, made it a formidable calculator for the engineer and scientist calculation, when he was far from a computer. I still use it every day and the search statistrics of my site still show it as the most searched item - after all these years.
The manual will describe the machine, list its several versions, explain how its firmware can be upgraded, and will list many programs for all areas of scientific knowledge and for all educational levels. Most of the information is already available if you know how to search for it, but I have missed a place where you can read all of it with a consistent writing and enough explanations on what each program is doing.
You may say that it does not make sense to write a book about a discontinued calculator, but I know for a fact that there are many of them still in use around the world, with peaople searching for software and longing to read more about it.
Besides, I do not lose the hope of having it produced again...