Here are two snippets from the Guardian review. First, some arguments on why authors concealed their names when they went to publish:
He's not interested in works of which we just happen not to know the authors, but in writers who consciously sought to conceal their names. Why did so many people do it? Mullan advances a wide range of reasons, from mischief to modesty. Alexander Pope printed An Essay on Man anonymously in order to trick his enemies into praising him. Mary Ann Evans used the pseudonym George Eliot because "a masculine pseudonym was a claim to authority", and perhaps also because her complex marital career made her feminine name more than a little uncertain. Dodgson, the Christ Church mathematics don, refused to admit that he wrote the Alice books because "My constant aim is to remain, personally, unknown to the world." (link)
There's also the question of why it is we place so much value on authorship:
Why do we need to attach authors' names to books at all? Doing so makes life easy for librarians of course: just imagine arranging all those novels ascribed to "A Lady". Having names on books also helps us recognise works which we're likely to enjoy ("the new Ian McEwan"). It also allows for the simple human pleasure of piecing together an author's interests through their oeuvre, and feeling that you know how they think.
But looked at from a wider historical perspective these are quite recent pleasures, and they don't have entirely innocent origins. When Henry VIII proclaimed in 1546 that the names of printers and authors should appear on all published books, it was not because he was burning to read the latest heretical treatise. It was so he could catch and burn their authors and printers. And when present-day publishers put an author's name on a title-page they do so because an author is now something like a brand-name. Doris Lessing experimentally sent The Diary of a Good Neighbour to publishers under the pseudonym "Jane Somers", and it was repeatedly rejected. When published it sold about one 10th as many copies as "the new Doris Lessing" would have done. (link)
In the end, our current emphasis on the identities of authors is a result of the dominance of the marketplace.