ebook ??? your opinion please! | warrior forum - the #1 ... - korean dry skin care products

If someone has a website in a niche area like acne and wants to develop an ebook for it about 100 pages, is it best to stick to the niche topic of acne, or broaden the topic, covers general skin care?
Or can it also work, such as e-books on niche themes of acne, or e-books on broader topics such as skin care? Thanks!