Aviation ebook for beginners?
Although this is a question based on opinion, it might perhaps have some value as a community wiki so I've marked this 'answer' as such. I also borrowed one of Manfred's links. Feel free to append stuff to the list.


In terms of free download, I can definitely recommend the FAA Airplane Flying Handbook which can be found on the FAA web site. There are also a number of other handbooks there which are great reads. To be honest, I find this the best introductory book I know, free or otherwise.
If you're out for a good reference book, something like Anderson's Introduction to Flight will be good, but bear in mind this is hardly an easy read but does cover virtually everything you will need to know. An older edition seems to be available online.