(?) after a title means I'm not sure its a real book/publication.
Literature mentioned in Murder Must Advertise by Dorothy L. Sayers.
The Morning Star and The Evening Banner (?). I know the Star exists, but am not sure about the Banner. It may have at the time of publication (1933).
Non-fiction and reference:
The Directory of Directors
Bartlett's Familiar Quotations
The Globe edition of the Works of William Shakespeare
The Children's Encyclopaedia
The Times Atlas
Brewer's Dictionary of Phrase and Fable (as Brewer)
Alice's Adventures in Wonderland (mentioned as Alice in Wonderland) (by Lewis Carroll)
In addition, P.G. Wodehouse and Edgar Wallace are mentioned, but none of their books.