recording interviews

I will be interviewing people for a book, and may compile and sell some of the recorded interviews.

Questions please:
1. Is there quality enough equipment to do this over the phone? If so, suggestions?

2. Should I instead use a go between recording service to interview over the phone. If so, using what technology?

3. Should I just interview in person if I want to resell the recording?

4. Should I use cassette or digital?

I really appreciate any advice.

Jill