Professor Challenger in Space, available for free download from Feedbooks.
Arthur Conan Doyle's great science-hero, Professor Challenger, sets off on new and terrifying adventures across the galaxy! First he gathers together the old gang – Professor Summerlee, Lord Roxton, and the journalist, Edward Malone – and then they are off on the one journey they never before undertook – into space! They meet unusual new friends, encounter vicious new enemies, and at least one of the party falls victim to violence! Another may well fall in love! And the others are sure to talk, argue, drink whisky and smoke cigars, if nothing else!
Though I'm much prouder of the things we're publishing nowadays, I have to admit I still have a soft spot for this silly book.
Update (26/9/12): after a couple of years - and over four thousand downloads! - we've taken this book down from Feedbooks. You can now find it on Kindle. Here it is on Amazon.com, and here it is on Amazon.co.uk.