How about an article first or a website?
Might feel a lot less intimidating. It'd be like Tae's Kim Grammar Guide for English speakers to learn Japanese. You can organize it similar to a wiki guide and then, if people respond well to that content, then you can consider turning it into a proper book.
Though I would say that you should probably invest more time into the needs of Spanish speakers looking to learn Chinese. That may give you some more focus (ex. business, travel, culture, etc.).
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This is a good idea. Indeed, I wouldn't dare tackle a whole book without getting some gradual feedback.
It may also be a good way to start actually writing, instead of waiting until after I've read a number of chinese grammars, plan an outline, etc etc.
Thank you for the input!
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