A library grape is a genetically-engineered fruit that contains gene sequences for producing any and all compounds responsible for fruit aromas and tastes. Particular sequences are expressed based on the various environmental factors impinging on a vine: sunlight, temperature, rainfall, pruning and tilling, etc. Library grape cultivation is more of an art than a science.
The Concent of Saunt Edhar's is noted as being a good place for cask-aging library grape wine, if not for growing the plant itself.
Here's how the library grape is described on Page 175:
The library grape had been sequenced by the avout of the Concent of the Lower Vrone in the days before the Second Sack. Every cell carried in its nucleus the genetic sequences, not just of a single species, but of every naturally occurring species of grape that the Vrone avout has ever heard of -- and if those people hadn't heard of a grape, it wasn't worth knowing about. In addition, it carried excerpts from the genetic sequences of thousands of different berries, fruits, flowers, and herbs: just those snatches of data that, when invoked by the biochemical messaging system of the host cell, produced flavorful molecules. Each nucleus was an archive, vaster than the Great Library of Baz, storing codes for shaping almost every molecule nature had ever produced that left an impression on the human olfactory system.
A given vine could not express all of those genes at once -- it could not be a hundred different species of grape at the same time -- so it "decided" which of those genes to express -- what grape to be, and what flavors to borrow -- based on some impossibly murky and ambiguous data-gathering and decision-making process that the Vrone avout had hand-coded into its proteins. No nuance of the sun, soil, weather, or wind was too subtle for the library grape to take into account. Nothing that the cultivator did, or failed to do, went undetected or failed to have consequences in the flavor of the juice. The library grape was legendary for its skill in penetrating the subterfuges of winemakers who were so arrogant as to believe they could trick it into being the same grape two seasons in a row.