We do not really use the second construction to express the future in English. This type of construction is used to talk about the future in languages such as Spanish and Portuguese. In English we say either 'I am going to read this book tonight,' as you rightly say above or 'I will read this book tonight.' 'I am reading this book,' is the present continuous tense and shows that you are reading the book *right now* rather than that you will read it in the future.

The future tense is "I will read this book tonight". Yours is present continuous tense. The differences in nuances are, I am reading this book tonight -- this means you are doing it right now. I am going to read this book tonight - means that you haven't done it so and perhaps will happen tonight.

The main usage of the present continuous is to describe actions taking place in or around the present. However, we do use it also to refer informally to things that you know you will be doing in the near future. e.g. What are you doing after work tonight? I'm meeting my friend for a drink. You could also say: "I am going to meet my friend for a drink." When talking about plans and arrangements in the near future, these two forms have the same meaning. Murphy is always correct! Never doubt Murphy! I am his biggest fan! Seriously, Murphy explains it very well in Unit 19 (3rd edition).

@Michael Chambers... There are four uses of present continuous tense... 1. To express something that is in the progress right now or not. I am giving some examples now. 2. Indicating a longer action which is in progress now but NOT at this moment... I am studying to become a lawyer; I am reading a book 3. Indicating something in the near future I am meeting some friends after my chores. 4. To indicate repetition and irritation He is always coming to work late; he is constantly chattering, I wish he would shut his trap; they are dislike because they are always whining away. So without context and getting into the nitty-gritty details, we just give the simplest version.