Rip off or creative inspiration ?

I am really curious about the design process, how each of us approaches a new design or addresses a design challenge.   At this early stage in my design degree, I don't allow myself to look at examples first because I don't want those images to influence my thoughts and ideas.  Once I get into a project and have solid comps, I may then take a look at at other designs. This brings me to the question above, when we see a design that we can gather ideas from and improve or take in a new direction, at what point do we cross the line between rip off or simply using another's design as a launching point for creative inspiration?  Please share your design process and thoughts.   Also, as part two, share a book or source of your inspiration. Selfishly, I am wanting to expand on how I approach design and would love to learn from the creative and unique minds in our class..so am I ripping off or simply getting inspiration?  A book I am happy to share for design inspiration is Feng Shui by Stephen Skinner.  I feel that design is design regardless of how it's applied - interior design, architectural, fine arts, or in communications are the same.This book was recommended to me years ago because of my interest in Feng Shui.  Enjoy!