The copyright status for works published in the United States between 1925 and 1968 varies depending on whether certain "formation" requirements were met. For instance, a work published in 1928 without a notice of copyright would fail to meet the legal requirements for copyright in that year and, as such, would be a public domain work.
Similarly, a work that contained a copyright notice, and was registered with the US Copyright office in 1928, but the copyright was never renewed in the 28th year after publication would also be a public domain work.
How do you know which "formation" (such as copyright notice and renewal) requirements were required in which year?
A few good sources include:
Let's say you've determined that the book was published in 1928 with the appropriate notice and registration. How can you tell whether the registration was renewed? Go to Stanford's handy copyright renewal database.