For bookplates and prints the BM catalogues are the standard reference works.
All three volumes of the Catalogue of the Franks Collection of British and American Bookplates in the British Museum Department of Prints and Drawings are now available online, thanks to a digitisation initiative by the University of Toronto.
The museum is putting increasing numbers of prints and ephemera online through its Collection database. At 21/11/11 there were 18,626 entries online under book plate.
There are 31,899 political prints online (which I found by entering satirical prints in the free text search).
The invaluable catalogue: Stephens, Frederic George; George, Mary Dorothy, Catalogue of Political and Personal Satires in the Department of Prints and Drawings in the British Museum, 11 vols, London, BMP, 1870 is also available online through the University of Toronto, together with a wealth of other reference works relating to prints at the museum.
The BM's collections of British 18th and 19th century trade cards (the Heal and Banks collections) are unsurpassed. There are currently 14,754 trade cards online.
There is also a new online research catalogue of the museum's Paper Money (nearly 20% already online), with articles on banking history, security printing, etc.