For a long time people have wondered about the possibility of life elsewhere in the universe, and in particular on Mars. In The rivers of Mars
Piers Bizony surveys some of the evidence for such life. The first part of the book looks at what was found by the Viking landers, followed by what we have learnt from meteorites from Mars such a ALH84001. The later part of the book considers the more general questions about life in the universe. I've a feeling that the book might not be detailed enough for some readers - at times it seemed a bit rough and ready - but there's a list of further reading for those wishing to study the subject further.
The book was published in 1997, and so doesn't deal with the more recent missions to Mars. However, it was the Viking landers which were sent to search for life - later missions have had more limited aims. In particular they have been trying to clear up some of the confusion from the Viking results. This book gives a good account of how this confusion arose, and of the arguments within NASA about what they meant. So if you think that life on Mars has been pretty much ruled out, then you should take a look at this book to see that in fact the situation is much less certain.