I would say all, if you're interested. So far, FOTJ hasn't had any strong standalone novels. Outcast sets everything up. Omen introduces some key characters/antagonists. So does Abyss. If you had to skip a novel, it could probably be the fourth one, Backlash. Though I recommend reading it because of the character development. I also don't think Allies, the fifth book should be missed. A lot of important things, relatively speaking, occur in this book. Personally, Allies has been my favorite so far. There's also the Lost Tribe of the Sith e-stories, written by Johnathan Jackson Miller I believe, which serve as a prequel to the events. I haven't read those.
I will say that that FOTJ seems to flounder a little in terms of what the central theme/narrative/rationale is. It shifts and a whole new subplot starts in Backlash that blossoms in Allies and might become the central story for the remainder of the series. Unlike NJO or LOTF, FOTJ seems a bit more scattershot. I think the new subplot would probably be better served in a series of its own, instead of throwing it in the mix here, especially so late in the game with only four books left.
FOTJ already had some interesting ideas-Force psychosis, etc., Jacen's jaunt through the Unknown Regions and why that might have led him to the dark side, but it needs to flesh those out more. Also, more work needs to be done with Jaina. I really think the writers should've done a better job setting her up as Luke's immediate successor in LOTF. But she seems to be a secondary character yet again in FOTJ. The writers only seem to find something to write about her, in relation to her relationship with Jagged Fel. She's the Sword of the Jedi for goodness sakes. I would think that would be something like the Mace Windu of the NJO. She should be having a lot more badass adventures. To be fair, Jaina does get some face time throughout the series, in fact more than in LOTF IMO. I thought LOTF really focused on her more at the end. But even with FOTJ, she's only starts showing her skills in Backlash and Allies, to a lesser extent.
