Essay is particularly a test of your knowledge and analysis about various issues, so one cannot confine himself to a particular source for essay preparation. The extensive reading during preparation of compulsory and optional subjects provides a lot of useful material for essay writing. What one needs is to have an art of setting down that knowledge on paper. This requires abundant practice of writing. You should keep up the practice of writing on daily basis. Your exercise of writing is directly linked with your essay preparation: more you write more your skills for writing an essay will get flourished.

As far as basic tips for writing an essay are concerned, you should consult any good standard book. Mostly recommended books for this purpose are discovering or exploring the world of english. You may also find any other book as well for this purpose. For practising essays, it is highly important for you to select 10 to 15 most important topics and then write upon them after thorough reading and analysis. I assure you that no tutor or book can provide you as much help in this regard as your own efforts can. So, just carry on the practice of writing without any hesitation. send your written work here, members of this forum will help you a lot by pointing out your mistakes and encouraging you for good attempts.