There is excellent advice in this thread, all of it relevant.
But your product will be more successful the more you personalise it.
By this I mean, that the more of your own experience you can bring to the product - booklet or info manual - the better it will sell.
Sure, you can research practically anything now over the internet and produce a booklet on it. but it will only come to life and gather good comment (essential for extended sales), once you can speak from your own experience.
This fact is the most underrated in marketing today.
Because of the anonymousness of internet marketing, your personality - the way you come across in print, your background, your experience - is one of the most essential success factors.
And so yes - people will pay for the free information that you've gathered on the subject, providing it links to your strong 'web personality'.