Great piece by Rebecca Smart in The Bookseller on niche:
The migration to online purchasing of print books, and then to e-books, means that the buying of books is now about processes of search and recommendation, rather than browse and display, and this leads to a focus on specific interest areas and trusted authorities. If you publish for a wide range of interests, promotion of each individual book is becoming increasingly difficult and expensive.
Eoin Purcell's Blog 