You can safely finish The Force Unleashed
and skip the sequel, though by the end you may be wondering how the hell a Gary Stu-style fanfic somehow found its' way into official SW print.
Joking aside, I did see what George Lucas was trying to do with that one (not so sure he pulled it off). Avoiding spoilers, the theme of the book can best be summed up by these two quotes from Lord of the Rings
: "A treacherous weapon is ever a danger to the hand....A traitor may betray himself and do good he does not intend."
Or you could save yourself some reading time and just watch the movie
. The novel really doesn't add much of anything to the story.
You can do the same with Order 66
, I think. It ends about as you'd expect an Episode III tie-in to end, and as one of its' main threads only resurfaces about 60 years later (in-universe time, in Legacy of the Force
), you can easily deduce the fate of the rest of the cast.