I used to work with a major publishing house - usually you contact a publisher and pitch them an idea, whether its a first chapter, a pdf of a paper thats already been published (a good starting point for a non-fiction book), or a full already written book. If its non-fiction you better be one helluva subject expert or have more people than just you. From there the editor will asses the potential risk/reward associated. If they see enough potential it will be pitched to a higher up. from that point its out of your hands until they actually decide whether or not to go with you. From there - well thats the fun part 😀