As people most likely stated: Yes! Use bags in 
your cel books.

When something is archival it means that it 
conains no acids ect that would damage the 
cel/photo/printout (like discolour it). But it 
does not mean that the cel wont stick to the 
materials. Cel paint is really evil stuff, it 
will definately stick to the paper in the cel 
book, and even if you take the paper out, it has 
been known to stick to the plastic itself. So, 
always bag your cels before putting them in the 
cel book. Bags are cheap... so no real reason 
not to use them everywhere ^_~ Hope that helps.

Otaku Inc. British Division