If you are selling a book and it's online through your website that is ideal because you retain the customer data. For example you can email people again in the future the next time you release a book. But most authors list their books on Amazon and leverage that search engine for conversions. Which is perfectly fine. But you don't get the data through Amazon that you get from your author website. And you WANT that data. So you'll want to include things in the book itself that drive people to an author website and register.
To start, we're not necessarily advocating for the use of rebates. You'll want people providing their contact information because they want to, not because you're going to send them $5. BUT it is a real option that each author will need to decide for themselves if it's worth doing or not.
In this situation you'll have ONE unique rebate code delivered with each sale (otherwise resellers will ruin you). Once someone buys the book, if they go onsite to redeem their rebate you have their contact information and in exchange for that you send them some dollar amount.
Now I don't know the laws behind rebates exactly and I'm not sure of requirements to obtain a rebate, so talk to an attorney. But wireless providers have been doing it for years and it's a very established form of marketing.
One thing to keep in mind though is that you need to provide rebates separately from wherever you're processing purchases. You can't process these as refunds. There is this thing called a customer satisfaction ratio. Things like chargebacks and refunds negatively impact this ratio. Too many refunds and you may lose your ability to process sales. So what you need to do is send the money another way. Through a check or Paypal or something. This way it is separate from the purchases and will not harm your brand.
Remember back when magazines were the rage and every time you opened it a bunch of offers and cards fell out? Well we suggest the same thing. Including an insert for a "free" something like an additional chapter sent by email or 50% off of a live event, webinar, 50% off another one of your books, Patreon access for 6 months free, etc. These could do wonders for your opt-in rate.
This is all about getting people to give you their contact information, so the offer needs to include some value as you'll likely only have one impulsive moment to get people to sign up. If they don't opt-in and receive their registration then you'll never know who they are and never be able to get back in touch with them in a meaningful way. So make the offer incredible!
Now this is going to be available through your production company only. If they are willing to allow inserts to be included in the book then wonderful. If not, oh well. But I'd push hard for it because this is a best practice in the ecommerce space.
This one is pretty obvious. If you have a book about cooking steak, make an ecourse about DIY steak seasoning and plug it throughout the book. The best example of this is in the book The Ultimate Guide To Google Ads. The author has written and kept updated an amazingly thorough book about how to run Google Ads. Throughout the book he talks about how important copywriting is to the success of advertising. He'll give some amazing tips for writing great copy, then share the URL where you can buy his full ecourse on copywriting. I'm telling you it's masterful and if you need a more clear example of how it works, buy the book and see for yourself. The point here is that the book is not about copywriting, it's about Google Ads. His stress on the importance of copywriting creates the demand. You'll need to do the same thing. If your book is about learning to read a P&L, you'll want to build an ecourse on bookkeeping and stress the importance of organized bookkeeping throughout the book.This is absolutely something that you should do if you can.
If you've written a book with tools to be applied throughout the reading process. Then you know how important it is that people don't just see those charts and sheets in the book, but they'll need copies for the staff and business partners. This is accomplished through YOUR website. You simply point out that copies of these documents can be downloaded for free at yourwebsite.com/freedownload. This is without a doubt the easiest way to convert someone. It's not salesy at all. You're simply offering this information conveniently as they go through the process of applying your teachings.
So on this structure your challenge is in writing inspiring enough content that people want to take action. If you can get people motivated enough to make changes that they go to your website and download a copy to use at work then you should have no problem building an amazing list of customers that you can sell a second, third, and fourth item for years to come.
Two books that perform this option to perfection are Sticky Branding and Traction. Traction is like a billion dollar business now, so you'll do well copying their system as much as possible.