YOu are close to answering your own question if you feel you have value that your target market will pay for.
I just finished two ebooks:
Kick It Up A Notch Marketing - 25 High Impact Marketing Tactics for Printers
and the same type of content in an ebook entitled
Kick It Up A Notch Marketing - 25 High Impact Marketing Tactics for Realtors.
Both are 85-95 pages and sells for $37.
I have a networking workbook with about 60 pages that sells for $17 at www.networkingworkbook.com.
I think value is the key. If it takes you 200 pagess then thats the number you need. I see the range from 50-200 pages. There is no magic forumla so concentrate on your content and let the pages fall where they may.
Certified Guerrilla Marketing Coach
Co-Author, Guerrilla Marketing in 30 Days
Two Free Chapters available: