Though you owned the copyright throughout the work’s creation, you may have handed over your rights without realizing it. Standard publishing agreements transfer all copyright ownership from the author to the publisher.
The publishing contract defines how an author can use their own work. Contracts often restrict sharing the work in the classroom, passing it to peers, or even referencing it in your own future publications. In many cases, you may not even have free access to the published work, and would have to pay for use of it!
As explained further in UNC’s Copyright library guide, this is because copyright covers these rights:
Publishers’ ownership of a work’s copyright allows them to:
If the publisher did not have these rights, they would have to reach out to you to gain permission each time they wanted to print or otherwise use the work.
Negotiating the contract at the beginning of the work’s publication process is an important step towards ensuring you keep the rights you want. However, if you are very unhappy with your author rights, you may request the publisher to revert rights back to you, though this also means they will no longer sell the book for you.
This is called "Self-Archiving."
(Note: Self-archiving does not refer to posting for the sake of marketing your finished work on social media. It refers to posting for the sake of reading accessibility.)
Regardless of your contractual rights, if your work is published in a format that requires payment to read it (like a subscription journal), you may still have “self-archiving” rights to post your work--or some earlier version of it--in a freely accessible venue, such as ResearchGate. Almost all publishers have an author reuse policy. This policy could be posted, for example, on some variation of the publisher’s website Authors Resource Center page, or on the publisher’s website Rights & Permissions page; or you could ask your publishing editor about the policy. Another alternative is to look the policy up on the SHERPA/RoMEO database, which externally collects publisher policies about online sharing in free website repositories.
It might be of value to you to check the publisher’s policy on self-archiving prior to submitting or signing your work to preventatively ensure some copy of your scholarly work will be publicly accessible.
Tip: your publisher might refer to its self-archiving policy as "Green Open Access" or "Green OA"