What would you say is the "minimum viable product" for something like this? I was thinking of writing a book as an intro to project management, but was unsure whether I'd need to do 50 pages or 500.

Remember back in grade school when our teachers would require that we write 20 page reports double-spaced?

We all knew that was bullshit, right? Same applies for books.

As long as the amount of value derived from the information outweighs the cost, it's a worthwhile purchase. Don't worry about the length. Write what feels natural, price it accordingly, and slap a 100% moneyback guarantee. If someone doesn't feel it was worth the price, don't take their money.

Thanks! :-)

I don't think the length matters much. Think of it more in terms of how much value you can provide in those pages? If it's 10 pages but makes me $100 more a month then it's a pretty good purchase.