Hi Viv,
The step by step series is indeed not enough for the expert exams. I find that the inside out series covers nearly all you need to know for the expert exams. Be aware though that there is a difference in versions (2007 - 2010). The backstage feature has
some additional options which lack the big office button in 2007.
I wrote a piece about prep materials on my blog you will find this under MOS Certification and if you look under books you'll find a sample chapter for excel as well
hth
Maurice