I got the first one for 20p as well. Twas great because I'd started the hardback and it was a pain to read in bed.I did find it went on a bit though. I shall still give this one a go as Ken follet usually does a good historical novel.
Here and here. The 20p ones tend to fairly well known authors using this as a way to promote their books, They will only be 20p for a day or two. The eReader site will send you daily e mails listing free books in whatever genre you choose. Most of the freebies are rubbish vanity publications IMO but there are some proper authors who have cottoned on to the fact that giving away a few free editions raises their profile and makes good advertising.