hi anniep,
that's a question which comes back regularly and the answer is a bit complex.
if you are reading drm-free public domain books, these can be shared freely with anyone you like, and providing the people you are sharing with have a reader compatible with that format or can convert the file to the appropriate format. you can get these books from project gutenberg, feedbooks.com, manybooks.com, right here on mobileread, and many other sources.
if you are reading books still under copyright, this is where the question becomes more complicated.
first, the question of sharing the books on a legal / ethical / philosophical level. many publishers / vendors would like to consider their sales as 1 file = 1 user, even though as you point out this is not the case with paper books. some of them have specifically allowed a certain amount of sharing ; for example, 6 sony devices can be registered to the same account, and all of the registered devices can access all the books on that account. epub files encrypted with adobe adept drm can be authorised for 6 devices. some vendors, like Baen, sell their books drm-free and encourage sharing with immediate friends and family because they consider that fair use
and free advertising.
most people would agree that sharing a book between 3 people from your immediate circle of friends or family is perfectly acceptable under the concept of "fair use", however to do this, if their readers are not registered to your account, you may have to remove the drm, which may or may not be technically illegal in your country.
on a technical level : files can be shared with other readers if they are drm-free (or have had the drm removed, which is possible for most forms of drm but may be technically illegal where you live) and a compatible format. if they are not a compatible format, most formats can be converted to other formats, although the quality of the results can vary.