Writing a book is not that difficult. Sure, it is laborious, but if you merely keep typing away day after day, eventually you end up with a manuscript. Writing a book that is "good" or one that other people would want to read is a lot harder. Still, even that is easy compared to trying to get a book published. According to various unreferenced sources on the Internet, out of all manuscripts submitted only 1 in 1000 to 1 in 10 000 gets accepted for publication. Probabilitywise this is roughly as unlikely casting five dice and getting six with all of them.
Having written a manuscript I went about tying to get it published. The common approach in most countries is that first you have to pitch your manuscript to a literary agent, and if you succeed, they will then try to pitch it to publishers. In Finland the the procedure is simpler, anyone can submit their manuscripts directly to book publishing houses without a middle man. While this makes things easier, it does not help with deciding how much the manuscript should be polished before submission. The more you polish the bigger your chances of getting published, but the longer it takes and the more work you have to do if the publisher wants to make changes to the content.
Eventually I ended up with a sort-of-agile approach. I first gathered a list of all book publishers that have published scifi recently (there were not many). Then I polished the manuscript enough so it had no obvious deficiencies and sent it to the first publisher on the list. Then I did a full revision of the text and sent it to the next one and so on. Eventually I had sent it to all of them. Very soon thereafter I had received either a rejection email or nothing at all from each one.