What to do (in the first instance):
1. Get rid of all actual references to its PhD status (yes it sounds obvious but it's a good start and there are more than you would think), eg. the abstract (put that aside for transformation into a book proposal).
2. Weed out some of that repetitive signposting, eg. 'as I prove in chapter 5', 'as I have just proved' – it is crucial PhD architecture, demonstrating to impressed examiners that you have an argument and are sticking to it, but highly tiresome for the general academic reader. A well-written book will not need to keep proving that it is arguing something, it will just be doing it, and the interested reader will have chapter break-down and index to keep them navigating around your text.
3. Embed the references to critical literature more naturally within your argument: the rather crude form of the critical survey or literature review, so necessary to the PhD, is perhaps the single thing that will need most work. You may, depending on the publisher, be able to retain the chapter order of your thesis with the critical survey (presumably) first: even so, try and make it more appealing, less like those 'crags cranks climb', a sort of sheer academic cliff at the beginning of your book. You will make your work more publishable if you are able to integrate the critical references more naturally as you unfold your argument. This may mean that you do not cite all the works you initially surveyed (you can still include them in the bibliography) or that others get relegated to footnotes and spread about the book. Nonetheless, this is an essential first step.
4. Make sure that it is clear: your adopted or developed methodology and theoretical framework may be over-familiar and crystal clear to you, but your readers may not find it so – you will help your book and your readers if you try to express even the most complicated theoretical relationships as clearly as possible (yes, Lacanians, we want to understand as well as admire you!).
5. Try to make the style smoother and more elegant. A PhD is not awarded for style and it is in any case difficult enough to gather, arrange and argue through your research without having to worry about expressing yourself beautifully. However, style can reel in or repel readers (and publishers) in a very immediate way so try to polish your text as much as you can.