Actually I don't think the problem is using a reserved word. Using
"library" for a sheet  name doesn't cause any errors when trying a simpler
formula like =Library!C10+Sheet1!C8.

In any case  I found a roundabout to my problem. Aparently If I store the
sheet name in another cell  and then reference that cell in the formula it
works! (If anyone knows why please tell us)
I also made  the row  number column  on the initial sheet instead of library
sheet, so now I can do without the vlookup....
so my working  formula looks something like this
=INDIRECT(S4&"C"&R9) where S4 = Library! & R9= Row number