[Avodah] prosbul question

[Lisa Liel via Avodah]

A friend of mine posted this question on Facebook:

 > Having failed to make a prozbul, I just spent some time looking 
around at books and online. I see no
 > discussion whatsoever on whether the concept of shemittat kesafim 
applies to in-kind loans, e.g. books
 > I've loaned out. Does anyone know of a source that addresses whether 
the shmittah year cancels an
 > obligation to return a loaned sefer, kli, whatever?

I have no answer for her.  Does anyone here?

Thanks,
Lisa