first things first, out of the countless Alexander-biographies out there, is is most probably not the best one. however, it's certainly one of the most acessible ones.
i've made the effort of reading this biography before starting with Mary renault's Alexander-novels (which i am extremely looking forward to now!) in order to try to determine how fictional her approach to biographies is. seince most of the first-hand sources on the live of Alexander are lost and all of what we call primary sources today are also second-hand sources who possibly already altered history, pretty much all biographies we have now - primary or secondary sources - are heavily muddled by the respective author's interpretations and this is something that should never be ignored when reading any text on Alexander, never mind whether it's Arrian or Renault.
Renault draws a very romantic and accessible picture of Alexander that might or might not be true but it's certainly very credible. all theories she offers the reader, she bases on ancient sources as well as her own understanding of history and hellenistic culture and society which seems to be pretty vast.
personally, i think this book is a good starting-point to start reading about Alexander and the complex history he left behind - it has a pretty good ancient source appendix in the back that everyone can use to do some further reading.
the book itself would deserve 5 stars for being one of the most enjoyabvle "history" reads i've had in a long time, but i'm sticking to 4 stars until i'm done with Renault's fiction.. i'm not quite sure yet how much of her novels went into her characterization of Alexander in the biography as well and this is something i'm a bit weary and scared of.