It would not be considered public domain, but you still may be able to use portions of the textbook.
It would not become public domain if the textbook was published with a copyright notice. Material published in 1976 with a copyright notice has a copyright term of 95 years after publication date. However, if it was not published with a copyright notice, then it would be in the public domain, and you could scan and use as much as you want. Please see this Copyright Duration Chart, and Copyright Term Calculator.
However, even if the copyright is still in existence based on the above, you may be able to use limited portions of the text book under the "fair use" doctrine of copyright law. Under the "fair use" doctrine of the copyright law, you are allowed to use limited portions of a copyrighted work for the purposes of criticism, comment, news reporting, teaching, scholarship, or research. This is embodied in the copyright law at 17 U.S. Code § 107, which provides as follows:
§ 107 Limitations on exclusive rights:
Fair use Notwithstanding the provisions of sections 106 and 106A, the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.
Your use would fall under the teaching (and possibly the scholarship and research) provision above. So, you would be allowed to scan limited portions of the text book.
Unfortunately, there is no set numerical amount which is considered "fair" as each case is based on its own facts using the four factors listed above.
So, in using the text book, you should only use as little as possible to avoid any copyright infringement claim.
I hope this answers your question.