You say you are retiring and then going back to work for the same company 2-3 days a week. There is a potential concern that first of all you won't be considered to have retired. I don't have enough information to give complete answers here. I strongly recommend you seek the assistance of a professional tax advisor to make sure you set this up right. Assuming you are in business, then you should track all expenses that relate to you earning income from your business.
Any vehicle expenses, stationary, computer expenses, subscriptions, professional dues, etc.
And as for a book, I would recommend my book, Beat the Taxman. It is meant to help small business people like yourself. This is a really good question, one I get often. There is no rule of thumb concerning percentage of time dealing with one client. At least not that is published that I am aware of.