There are some great websites and magazines available on London. My favourite site is
Londonist and this has stories etc on topical issues.
There are lots of sites dedicated to London and the hugely cosmopolitan nature of this fantastic city (and some of the squillions of things that you can do while you are here).
If you are going to buy only one guide then I would suggest you look at the Top 10 London guide as a very good option. It's got info on all of the places you may want to visit plus top 10's by city area.
There are lots of ways to see London from the hop on hop off buses to using the tube. If you have never been to London and have really limited time then the HOHO buses are not a bad option unless it's wet. Once you know where you are going then the tube is a faster and cheaper option - unless there is a strike or something.........