Read and understand the terms of your contracts.
Understand copyrights. This includes art, photos and all other intellectual properties.
Understand that you as an author are making a percentage of the NET monies generated by your book. The rest of it is not yours. It belongs to the publisher (or if self-pubbed, Amazon and / or Smashwords, etc) to do with as they wish. Usually this goes to infrastructure, salary and other payouts.
Get up to speed on when certain payouts happen as they might change based on which third party is issuing funds. These are very different depending on where the monies are coming from (Amazon, non-US Amazon, mass-market distribution, audiobooks, brick and mortar bookstores, etc.) Know when the quarter payouts hit and who is paying them out.
Become aware of any opt-outs offered to you. Ask what resuming your full rights are. By law, the publisher owns everything other than your submission document. Some publishers might require a buyout in order for you to get your formatted and edited document and your cover art. What this means is that you’re reimbursing them the costs of producing your book. Some publishers are generous and will revert your rights to you and give you the final edit and cover or just the final edit. You may or may not get it formatted for publication as an ebook. Buying out audiobooks and translations will cost an additional amount. Be very aware of what you want back and what you want to pay for.
Understand the timelines required for publishing a novel and other pieces. As well as the timelines to take these down from third parties. Usually this is 30 days or sometimes more.
Why am I bringing this up? Because when you understand what your contractual rights are for your intellectual property, you have a firm knowledge of what your options are.
I’m not advocating, endorsing or speaking against anything with this post. This is simply a reminder to examine your rights and know what you’re getting into or out of.