Before commenting, you should read the section Why We Offer Free Downloads. It explains what the authors are looking for. Here, we speak only of form and courtesy.
Writing is a tough business. It involves long, lonely hours working on something you hope will give a small pleasure to others, and fearing making a fool out of yourself. When you, the reader, see something you like, please say so. It will cheer up the author beyond your belief, help others to decide whether or not to read the book, and will help the editors decide what to do with the authors next work.
Of course, the comments don't all need to be positive as the most valuable comments are those pointing out what the author can improve. However, they should be honest, specific and helpful. "This SUCKS" is not a helpful comment because it does not give any direction for the author. It simply vents the childish displeasure of the reader. Remember, this is somebody's baby that we are discussing here. It is far better to point out that a child could use braces rather than to say the kid is just too ugly to live.
It is appropriate to tell an author such things as some or all of the plot is not believable, or that one or more the characters does not act rationally or believably. I have, myself, told an author that his style was as dry as a textbook, that I could not see any definable plot in a writing, that all of the conversations in the book were stilted and uncomfortable, that he should occasionally spell some name in the book correctly, and that no one was going to read a book about a giant robot who appears magically at the window and helps a geeky teenager secretly destroy an evil alien race trying to conquer the Earth to eat humans for dinner. As I said, not all comments have to be positive, but they all have to be specific attempts to clarify the problems.
The more specific a comment is, the more helpful it is, and the more the author will appreciate it. If you just can't buy the idea of a nuclear powered bicycle, or notice that the author has repeated the phrase "Golly" way too often, or notice that he has placed Greenland near the South Pole, please let him know.
Frankly, the author has spent many hours writing the work that you read, and it is fair for him to expect you to spend a few minutes giving him feedback. Think of it as doing your part to make certain that we all have good books to read next year.
Senior Editor, Great Authors Online