Jacob(us) or Iacob(us) are the Latin form for both Jacob and James. Saint James is latinized as Sanctus Iacobus (or Jacobus). The cognate form Iacomus is only present in late Latin or in vulgar latin. In dutch there are Jacobus or Jacob used for the English names James and Jacob. The pet forms are: Jaap, Jaak or Sjaak.
A guy I know has this name and it really suits him, I find it a nice, strong, masculine name!But, everybody when they read his name out missays it. Every. Single. Time. They go Jack-O-Bus, Not Jah-Co-Bus.