Sadly, we have lost a lot of the writing that existed in antiquity. There are hundreds of works we have in incomplete form or only know about due to mentions in other works. However, I'm not sure if there is a realistic way to estimate the percentage . But, to my understand, the vast majority of works were lost. Before the advent of Gutenberg's priting press, all books were handwritten. Thus, mass production was not feasbile. This increases the probability of every copy being destroyed.

Whether they are a "desirable" selection would really depend on the quality of what was lost, wouldn't it? A lot of times what survives is just down to pure luck. Granted, societies often perserve what they find valuable and papyrus can be perserved in the Mediterranean climate of Egypt, Greece, and Rome.

As to whether you would recognize work sin a library in 100 AD also depends on when the works were actually lost. This is also difficult to know. Many of the works were likely lost in antiquity. However, later destruction is possible.

However, provided your in the Roman world, you would most likely find a lot of recognizable Greek writers. The Romans were, in general, avid lovers of Greek culture and nobles often sought out Greek teachers for their children.

Unfortunately, we will never know exactly how much has been lost.