http://www.lonelyplanet.com/
Lonely Planet has offices in Melbourne, London and Oakland, with over 500 staff members and 300 authors.  They have produced over 80 million books!
http://www.roughguides.co.uk/
Rough Guides have travel guides to hundreds of destinations worldwide, so start planning your next trip!
http://www.bradt-travelguides.com/
Bradt publish guides to over 100 destinations. From the seriously offbeat to popular tourist locations the guides are written with knowledge, sensitivity and understanding.
http://www.footprintguides.com/
Footprint have been publishing travel guides for many years and can provide you with a guidebook for every sort of travel; from a city break to a round the world trip and everything inbetween!