Bookshops in Cambridge, UK

I am going to spend a day in Cambrigde soon and I would like to find some good bookshops. I am looking for something similar to http://www.arielbooks.com.au/ or http://www.berkelouw.com.au/about/paddington in Sydney.
Any hints would be highly appreciated.

Cambridge, Boston or Cambridge England?
England : http://www.cam.ac.uk/cambarea/local/bookshops.html
US : here

Heffers.

Trinity Street.

sorry I mean Cambridge England, although I would love to go to Boston. Thanks for your replies.

Yup, Heffers, or also Borders on Market Square. There's also a WHSmith on Market Square or a Waterstones up at the Grafton Centre.

If you turn off Market Square into Sidney Street (fighting against the stream of studenty types clutching Sainsbury's bags), you'll find an Oxfam charity bookshop and also Galloway & Porters which are good for browsing and you can sometimes get more unusual books pretty cheaply.

Cambridge University Press also has its own bookshop on the corner of Trinity Street/Market Square if you're looking for academic books.

Leah has it pretty much covered. But will second the recommenadation of Galloway and Porter on Sidney street as they often have good second hand book that are difficult to get hold of otherwise.

I have always found the staff in Heffers incredibly helpful - though its not cheap and over the past few years has been turning more and more into your average commercial bookshop (it was bought by blackwells in 2000 but still keeps the Heffers name if I recall correctly)

Another recommendation for Galloway and Porter, which is indeed fantastic for browsing. http://www.gallowayandporter.co.uk/bookshop.html
The website says they concentrate on "reduced, remainder and second-hand books", so their stock is always changing, but I don't seem to be able to step in there without buying something.