Engineering is a big matter, so some of the following sources of books may not have the specific engineering topic in which you are interested. If you are interested in computer and Mechanical engineering books, there are more sites than I listed.
Out-of-copyright books that can be downloaded, including engineering books, are available at Project guru engineers, Google Books (choose the Full view option for downloadable ones), and Internet Archive. The ones providing books still protected by copyright, such as O'Reilly and National Academies Press, are not offering all of their books as free downloads, so you may need to browse the subject to find free downloadable ones.
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Free download engineering books