Some books are "locked" using DRM, digital rights management software, and can only be read using software that confirm that you have paid for the right to read the book.
Some books are unlocked, but stored in a format (there are lots of formats...) that only some software can read and display.
Many here unlock/crack DRM, but it is not something we discuss here... Because it can be seen by some as illegal, a form of theft, unethical or simply bad(TM).
In another part of this forum you find the software "calibre" discussed. It is software to manage (mainly) unlocked collections of ebooks, and read them. calibre can display most eBook formats, and can also be used to do an incredible amount of other things to eBooks and collections of eBooks. For instance convert an eBook from one format to another. I really recommend you to use calibre to read books on your PC, even if it may be a little complex and daunting to start with...
it would be easier to recommend free or commercial sources of eBooks if you give some indications of what type of books you like. I think you can find a lot of other suggestions here.
Sources of "free eBooks" on the internet are giving away unlocked/cracked "stolen" books. This is obviously unethical. Some may do this to get you to look at ads or to get an opportunity to infect your computer with malware. Beware!!!
But some sources are legit!
For instance this forum has a huge collection of really free books you can download. It is typically older books where there is no copyright left. But it is also often great literature, lovingly hand crafted into fantastic eBooks. Without any DRM or other restrictions.
Baen sell and give away a lot of great science fiction and fantasy books. And they even have a "real" free library of eBooks.
http://www.baen.com/catalog/category...ibrary/id/2012
They give away the first part of a series, or a few early books of an author, in the hope that you then be hooked and later will buy more books in the series or from the same author. Still, it is a great way to start a eBook-collection if you are into SF/Fantasy. And you can download the books in different formats for free. And best of all: The books are all without DRM!!!
I suggest that you download and install calibre and then download some free books from this forum or Baen (or some other site with free eBooks) and try it out.
I like the epub format a lot... It is used as the "base" format by calibre. That means that you can easily use calibre to convert epub eBooks into any other formats that calibre supports. So I recommend that you start downloading epub-books.
Software: calibre
Free books: This forum or legit sources on the internet!
EBook format: epub, to start with.