And "ancient books don't exist" means "ancient books don't exist". As one thing, it is the object of "sadness". What is sadness? It means "ancient books don't exist" Because "ancient books have not been preserved" is an independent sentence, which cannot be used as an object, it is as meaningful as adding "this matter" in modern Chinese to cancel independence.
"Fu" can be understood as a pronoun, similar to this and that. "Mourning for the non-existence of ancient books" means: lamenting that these things have not been preserved in ancient books.
The word "with" of "with" is a conjunction, equivalent to "so" and "from".
We put the above explanations together, which means that because the stone tablet with words collapsed, we are very sad that the ancient books did not save the contents recorded above. Simplified translation can be expressed as: lamenting the ancient inscriptions that have not survived in ancient books.