I have both these problems right now. On one hand I have had The Strange Angels book #3 and 4, Jealousy (out in 2010) and Defiance (out in 2011), on my shelf for ages and I wanted to finally get to them and read them, but by now I don't really remember every detail in the story because it's been a long time coming.
On the other hand I just bought Lost in Time (Blue Bloods #6) by Melissa De La cruz and I kind of think I should re read it all just so I have all the stories straight since it's a pretty long series and I am sure some sort of detail must have escaped my mind by now.
If you were in my position, and I am sure at one time or another you have been in this position, what would you choose to do? Re-read the series or just go ahead and read the book and get caught up as you read?