One of the few common factors among every PhD student's experience is the sheer wealth of reading that is required, especially in the first few months. This post contains 5 recommendations for essential reading that would have served me well before starting my PhD.
The illustrated guide to a Ph.D. This 5 minute read contains an excellent deconstruction of the PhD process, and helped clarify the exact purpose and nature of a PhD. Essential reading, and I send this link to anyone who wants to better understand what it is I am doing.
How to Win Friends and Influence People. I have serious reservations about this book's title, as it paints a rather exploitative picture which would appear to target the sort of people who think that friends are "won" and that influence is learned rather than earned. In actuality, the book is a set of short stories and anecdotes, tied into a loose narrative, that reaffirms the (largely common sense) importance of considering other peoples' opinions and interests in order to effectively communicate. A PhD is a very personal journey to becoming a world expert, the success of which relies largely on your ability to communicate and interact well with peers and superiors whose opinions and interests may not coincide with your own. This book does a great job of providing an explicit framework for improving the quality of these interactions.
The Elements of Style. The primary medium of communicating research is the written word. For this reason, the importance of the ability to write clearly, succinctly, and using good English cannot be understated. This book is considered by many to be the authority on good writing style.
The Ph.D. Grind. The PhD Grind contains a well written and entertaining account of one man's journey as a PhD student. It portrays an intimate story of their mistakes and regrets, and gives the reader a real understanding of what they went through. While everyone's experience is different, this memoir contains a unique insight which I found thought provoking and useful for contrasting my own experiences against.
124 research papers. Gaining a reasonable grasp of your particular field of research requires an exhausting amount of background reading. The only way to read the amount of papers which is required is by developing the skill of academic reading - the ability to quickly gain an understanding of the purpose, methods, strengths, and weaknesses of a paper, without having to perform a cover to cover read. While there are many guides around which aim to help develop the skill, I found the best way to improve was by practise. In my first month as a PhD student I read 124 research papers. Had I started this reading process earlier, I would have been able to hit the ground running at a greater pace, able to make quicker progress in those initial formative months in which you hone your research topic through extensive literature review.
This list reflects only my personal experience, but I feel that the recommendations cover a wide enough variety of topics to be a worthy read for any student preparing to start a PhD. There will be a lot of reading, so the earlier you start, the better.