Working from paintings (that aren't yours)!

Kate brings up a good question about working from someone else's painting. Read below:

Hello Joyce,

 I wanted to let you know how much I enjoy your book 'Big Art Small Canvas'.

It is one of my favorite instruction books. Your work really is wonderful.

 

Friends are encouraging me to sell the paintings I have finished from your book.

 

I am not sure if this is legal or not and have seen many differing answers. Some instructors say yes, some no.

 

What do you feel about this?

 

Thank you,

Kate

I told her it was the biggest complement anyone could give me! As long as she's not signing my name!
I've used other paintings as inspiration (and even my own paintings). What I usually do is title it "In Homage to " or "After". Since I work really small, it's obvious that I'm not trying to forge their work.
Does anyone have another way of handling this situation?