i haven't seen the book in question so i don't know whether or not you'd be copying something unique and original by using the author's design idea(s) from memory. if that would be the case, then your guilt is appropriate and i highly recommend you listen to it.
being unable or unwilling to pay for something we are not entitled to have for free does not justify taking it.
guilt is our brain's way of warning us that we are considering or have done something outside our ethical boundaries.
listen to your own heart and conscience. that's what you'll have to live with. why risk investing money and effort in something you won't be able to fully enjoy?