mr ploppy is right. It's going to be easier to get a zombie lover to buy a zombie ebook than a ereader lover to buy an ebook about zombies. Also got to bear in mind that people don't necessarily go to a forum to be sold a book. Both of these are likely to contribute to low sales I reckon.
Forum based promotion is a long slow burn I reckon - and better to think of it as a place you like participating in and may be you might sell the odd book or two rather than a promotional channel.
Alternatively if your book appeals to a specialist audience eg dog lovers than being known as a thought leader through your contributions on a dog lovers forum would probably garner you a lot of sales (and may be other business eg consultancy).
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