The iBook (Firewire) takes PC-66 SO-DIMM RAM. iMacs (the old ones) take PC-100 SO-DIMM RAM, and since PC-100 is backwards compatible with PC-66. Yes the iMac ram will work in the iBook.
I've taken ram from an iMac and placed it in my PowerBook G3 Lombard. Works fine.
I've taken ram from an iMac and placed it in my PowerBook G3 Lombard. Works fine.