If you've been developing your Arabic handwriting by imitating the printed script used in textbooks and other printed material, you're pretty much wasting your time. The 'Naskh' script is not used by Arabs in handwriting and attempts to imitate it only end up looking amateurish.
Mitchell's book is the only really good introduction into both the handwritten and calligraphic 'Ruq'ah' scripts that I know of. It has a very systematic approach, and comprehensive and thorough coverage which makes it relatively easy to locate specific details-- but since the individual sections are interdependent the student will have to work iteratively through the material front-to-back a number of times until it is completely mastered.
As alluded to in a number of reviews, this book is definitely 'old school'. It is a reproduction of a typewritten and handwritten manuscript, has a pretty high 'information density' (so the book itself is correspondigly deceptively small), and is unforgiving in its use of technical terminology, demanding focused attention from the reader.
Just to reiterate a point made by another reviewer, this book bears no relationship to Alif Baa at all. Alif Baa is a grammar tutorial that does not go into handwriting at all-- and vice versa, Mitchell's book presupposes quite some basic knowledge of Arabic orthography. In any case I'm surprised that someone would recommend Alif Baa under any circumstances, as that suffers, in my opinion, from a great many defects.
If you are used to modern typeset textbooks with four-color graphics, sidebars, and elaborate formatting, you may be disappointed by 'Writing Arabic'. If, however, you can take this book on its own terms, you should find it a valuable and reliable resource.