I think that all recipe books including the rare ones are numbered based on the level needed to scribe them. For this reason, the "normal" books include every number since there are new recipes to scribe for every level.
When it comes to the rare books, (like all the advanced ones), this is not necessarily the case. Given a lack of rare items for any certain level, there need be no book.
So a lack of an Advanced Scholar 13 just tells me there was nothing unique for a lvl 13 scholar to scribe.
BTW - I looked on
EQ2Traders in their selection of "recipes by book" and they didn't show one there either!