No! It is not an infringement to link to content that has been uploaded lawfully.
If, however, you have reason to believe that the content you're linking to was uploaded in violation of copyright, then you should not link to it. Doing so could be construed as contributory infringement. In those circumstances, work through the Step 1 questions with respect to use of legitimate copies of the content, rather than linking to infringing online reproductions.
For instance, one of the fair use factors takes into consideration whether your use is for non-profit educational purposes. Once you begin publishing a book and earning royalties, the commercial nature of the endeavor may weigh against fair use for that factor. You'll need to undertake Step 1 analysis again in its entirety when it comes time to publish your manuscript commercially, and determine whether you need permissions.
Keep in mind, too, that your publisher may--as a matter of policy, to protect itself--want you to obtain permissions irrespective of whether you believe use would have been fair without permission.
If they contained authored, original expressions, they were, and maybe still are, protected by copyright. But like any other copyrighted work, they may have entered the public domain.
Unpublished works subject to copyright protection. However, the duration of copyright for unpublished works can differ based on whether they are signed, anonymous, etc. For more on copyright length for unpublished works, consult the discussion of Unpublished Works (Ch. 3.2.1) in Peter Hirtle's Copyright & Cultural Institutions book; see also 17 USC §§ 302, and 303.
Keep in mind, too, that while unpublished works are not excluded from your use as fair use, what constitutes fair use of unpublished works may construed more narrowly by a court.
If you're looking to use foreign works in your dissertation being published in the U.S., the general rule of thumb is that anything first published in a foreign country prior to 1923 has entered the public domain, and most everything else published abroad since then remains protected by copyright.
The more complex answer is that, for foreign works: Based on the nationality of the author and place of publication, one can calculate whether the foreign material has entered the public domain. Though, you don't have to--you can use the wonderful Cornell University Public Domain chart prepared by Peter Hirtle. Check out the section "Works First Published Outside the U.S. by Foreign Nationals or U.S. Citizens Living Abroad."