You’ve probably searched for a book first published in (let’s say) 1909 on Google Books or a similar site, and discovered that they feature an edition published recently that only offers a preview and not a full view. The reason, supposedly: the book is “copyrighted” because that publisher has brought out that recently published edition.
The truth, however, is somewhat more complicated: yes, you would be taking a risk to make available to others copies of that recent edition of the 1909 book, because the publisher likely added special features like an introduction, commentary, appendices, etc., in order to justify its “copyright.”
However, if you have access to an edition of that book published in the United States prior to 1923, feel free to digitize it and make it available to others. Books published here prior to 1923 are now in the public domain, and saying that such a book is now under copyright does not make it so.
Note: Rules do differ from country to country—here is a nice list of copyright lengths by country: