Well, it's not like it's divided into "This book is C-level, that book is S-level". It's more like each individual reference in that book is defined as perhaps C-level, other parts are S-level, while some stuff might be completely N-level cause it contradicts a higher canon level. Some books even have G-level canon, which is the highest level of canon there is (stands for George if you haven't figured it out yet ), unless it's contradicted by the movies themselfs. Hence, while most of the Holiday Specials are considered N-Canon, it still has elements in it that is C-Canon, or even G-Canon. Anyway, this is getting very off-topic here, so if you want to read more about this, LucasForums has a very good explaination on it here.