There's a difference between illegal and actionable. Unless the damages would amount to more than some threshold amount that would push you over into the area of criminal copyright violation, it wouldn't be so much illegal, as actionable. Which may be a subtle distinction, but does have some moral shades of grey.

Actionable means that they can sue you for doing it, rather than your going to jail or having a criminal record. I hate to suggest the obvious, but I can't help myself... have you considered writing to the author or estate and asking them if it would be alright to do so?

The pro's are that if they say yes, you're golden. The cons are that if they say no, and you do it anyway, you can be sued for even more.