Pages are not a very useful concept for reflowable e-books, but the PDF version is formatted for a paper size that is common for technical books and contains just over 160 pages of useful text (that is 160+ pages not counting introduction, acknowledgement, index, etc.)
PDF (formatted on B5 pages = approx. 176 x 250mm)
ePub (reflowable text. Open e-book format. Every e-reader in the world has it's own quirks but looks good on Calibre, Kobo and Google Play Books. Aldiko looks ok, but tends to squash images a bit.)
Mobi (reflowable text. Amazon proprietary format. Formatted for Kindle Fire or better)
Because of the many code snippets and illustrations this is probably not a book for reading on a small phone screen. I would recommend the PDF version.
This book is for add-on developers who want to go one step further and add a professional touch to their creations or want to add functionality that isn't so straightforward to implement.
You should have a good understanding of Python and have some knowledge of writing Blender add-ons already. This is a cookbook in the sense that it provides small pieces of code that are aimed at solving a particular problem but if you want a more general introduction you could consider my book Creating add-ons for Blender.