my thoughts
Its a Wayist Document and they are independent of spires. If the book was not a public document it can be assumed it contained information the Wayist and /or some other power/organisation wanted kept secret/. Its existence would have been suspected and maybe its nature but not the contents. For example the Spirearch does not seem surprised when given 2 copies and recognises its importance. As far as we know Ferus showed no interest in it yet Cavandish did so its not necessarily something central to an etherealist. So means to an ends rather than having any power in itself and Cavandish was securing it for the Aurorans or for her Master. Its value it seems is in the content remaining secret now its in Cavandish's memory and in her physical copy the actual book she had is irrelevant. That Albion has it is of no concern. Also of interest given that she has memorised it she does need a physical copy. In her memory it would make her more important but she cant risk that or she wont defy orders to provide a physical copy.
Content is names clearly burning the copies is an attempt to delay the knowledge of the content from being known to others but not an absolute necessity or Cavendish would have destroyed her copy when memorised. The burning of the Monastery is a definite primary target of the incursion of the marines sending a message to others and acquiring the Index. Destroying copies useful but as subsequent actions of hers shows not absolutely essential.She wants more copies in her control as she orders for Marines to make copies. She is not worried about the marines seeing or knowing the names. Either because she is planning to kill them or because the names as such are meaningless to them. Some other knowledge or context is needed to make sense of the names. The index is some link in a chain. That the Spirearch doesn't open to read suggests he knows this is not immediate knowledge he needs or not immediately available to him simply by reading.
Names can be of things and of people. Which, really we have no clue and can be of both ie names of Spires and of people in them. From the mass of pages on her table on to which Cavendish was copying her memory the index was a book of some size and therefore a large number of names
By the way we only have Cavendish's statement that it contains names, is that all it contains, is she lying?
"An index is an indirect shortcut derived from and pointing into a greater volume of values, data, information or knowledge"
That its called an index suggests the latter part shortcut to information or knowledge. Names of other books people who hold knowledge or where it can be found. There is a suggestion of secret knowledge and not much of a stretch to associate that with the Builders and what will unfold as a race to its discovery.
