In copyright law of practically every country in the world, a book automatically falls under copyright, even if no license information is present. The right to copy is exclusively granted to the author, with a few exceptions such as the right to cite and in some countries the right to fair use.
The rights on a book don't have to belong to the author, though. For example, the author may have chosen to transfer (some) of the rights to a publisher.
The copyright may have expired
However, copyright doesn't last eternal. (A few lifetimes, but not forever.) If enough time has expired from the moment of publication, the book is now in the public domain and may be used freely. An example would be the works of Jules Verne.
Of course, those books weren't originally published as eBooks, but in many countries if something is an accurate reproduction without changes to the content this would not make it copyrightable because of a lack of originality.
The book might placed in the public domain
The owner of the copyrights on the book may have chosen to place the book in the [public domain]. In that case, you can freely share the book.
The copyright owner might have granted you the right to share the book.
Look for information about the copyright. Besides information in the book itself (usually found at one of the first few pages), you could look for license information at the web site where you downloaded the book.
A few licenses commonly used for content the author wants to be shared are the GFDL and all the Creative Commons licenses.
The laws of your country may allow you to share the book
Some countries may allow books to be shared with family, for educational purposes etc. It depends on what country you live in. You will have to consult with someone familiar with the laws of your country that are applicable, or look it up yourselves.
You are not allowed to share it
If none of the above is the case, you're not allowed to share the book legally. Whether it's morally okay is something you'd probably best answer yourselves.