If you're in the UK, then the Life+70 rule should apply. Basically just look up the author on Wikipedia, see when they died, and if it's more than 70 years ago, then the book will be okay for you to download under UK copyright rules.
Also, there are also more recent books which the authors have released into the public domain or licensed for re-distribution under Creative Commons, which will also be okay.
Here's Wikipedia's
List of countries' copyright lengths and a direct link to their entry on
UK copyright law for more info.