The British Library's online catalogues, www.bl.uk , are a useful resource. I find the English Short Title Catalogue particularly useful as it sometimes contains bibliographic pointers to distinguish different editions and issues of early English books.
The Library of Congress also has an online catalogue, http://catalog.loc.gov , but I find the British Library’s Catalogue easier to use.
Wikipedia, http://en.wikipedia.org/wiki/Main_Page , often gives the basic facts about an author or book. It saves getting out of your chair to find a reference book, although like most encyclopaedias it sometimes seems to tell me everything about a subject, except what I want to know.
I suppose should mention www.abe.com , which contains several million books for sale they tell us, but is fast becoming overwhelmed by print-on-demand paperbacks. A word of warning too regarding some of the amateurs offering books for sale on abe; sometimes they do not understand that the condition of a book has to be described fully and completely.
The 'Antiquarian and Collectable' section of 'Books, Magazines and Comics' on eBay, www.ebay.co.uk , is always worth a look in a spare quarter of an hour if only to laugh at the prices asked for books that belong in a Charity shop, and again the caveat about being wary of defective descriptions.
Finally if you want to check out my colleagues in Cecil Court, www.cecilcourt.co.uk  or if you have lost someone’s email or phone number visit the Cecil Court website,