Finding OA books in STEM may be harder to locate, but here are some good options to try.
Most of the publishers on the Open Access by Publisher list cover ebooks as well as journals (including JSTOR). You should be able to limit results to books/book chapters, though with some publishers you may still need to browse the results for OA content if there are no options to limit/refine by OA access.
The Directory of Open Access Books indexes more than 40,000 OA books from hundreds of publishers, though it skews heavily to humanities and social sciences.
If the book is in the public domain (e.g. published before 1926, government documents), you may find a free, digitized copy on HathiTrust.
In Google Books, you can limit search results to free books by going to Tools, then selecting "Full View" from the View options. There is a lot of overlap between HathiTrust, so you would be able to download more of the book. Some books are also partly available, so you may be able to access the page(s) you need even if most of the book cannot be viewed.
If you select Preview and Full View to include the preview-only titles, you may be able to view the pages you need.
