Wellington - a very broad question. I suppose the best answer that I can give - is when the idea is established and there is a clear market for the book topic. I would not advise 'fully' writing a book before you have first approached a targeted publishing house - and have confirmation to proceed. It can be a good idea to send in a draft chapter to allow the book proposal reviewers a 'hint' of writing ability, style and topic relevance. Chapter 1 is a good one to start with as it tends to 'set the scene and context' of the overall book.
Getting a book published is not easy - and much harder than getting an article published; there are many more people involved, contracts to organise, long-term planning and deadlines to meet, marketing etc. My advice is to draft as best a book proposal as you can and send it to your targeted publishing house topic administrators. Most established houses have their own online template that you can follow. Unlike article manuscripts - you can send your proposal (at the same time) to as many publishers as you wish - as long as you have not entered into a contract with any of them.
Wellington - The best time to publish your first book is soon after your PhD award. By the time, the researcher learns the necessary skills of writing and has acquired sufficient training. The PhD thesis can be converted into book form after making all those changes recommended by the experts and taking necessary permissions and by adding more chapters. After the draft is ready, contact many reputed publishers at the international, national and local level. Once a contact is established make further changes as per the guidelines of the publishing house. The important thing is haggling with publishers for free publishing, copyright and royalty.
It depends. If the author can manage single handed writing a book, well. Team work is usually good for publishing because work gets divided and many ideas can be generated. It is a good practice to divide work among co-authors in order to finish the work well in time.
It is better to publish with good publishing houses like Penguin Random House, Springer, Wiley, McGraw-Hill, HarperCollins, Sage etc. If this is not possible due to some reason, then it is better to look for reputed publishers at national and local level.