I did my LNAT last year, and those who say that you can't really prepare for it, I do not agree with. I practised using the tests provided on the website and got very good marks, but didn't bother to buy any books such as the one the above poster recommended for his son and my actual LNAT score was dismal haha. In hindsight, I definitely would have purchased the book.
When it came to the essay section, luckily all my A Level subjects were essay-based and I am quite apt at essays in general, likewise keeping up-to-date with current affairs wasn't much of an issue.
In short, I would advise you to buy the book, because from what I've heard, it provides many essay titles for you to do and I believe multiple-choice questions? (Someone correct me if I am wrong), as well as general advice.
Keep up-to-date with the news, read the quality newspapers too.
Also, have a firm stance on particular moral/ethical topics, which make it easier to shape a response to. I remember my LNAT essay question, (Should university education be free?), I felt at ease when it came to answering it because I 100% knew my stance.
They provide you with a whiteboard for the planning of the essay btw, so don't worry too much.
I wish you all the best with your preparations!