Harvard Library has many books available online. To find e-books, search for a title or keyword in HOLLIS, the Harvard Library catalog. Then, filter by "Show only: Online" and "Resource type: Books" from the facets on the right-hand side of the page. For books that are available through EBSCOhost you will be prompted to set up an account to download the book onto your device. 

If you only need a chapter from a physical book, consider placing a Scan&Deliver request from a book's HOLLIS record. Library staff will scan a section of the book for you and deliver it to you via email as a PDF.

Other Sources for eBooks include:

Internet Archive - The Internet Archive now has a 'Controlled Digital Lending' feature. They scan entire (copyrighted) books, and then lend the book out for two weeks at a time, digitally. To borrow a book, students can create a free account here. 

JSTOR - Temporary access to eBooks we don't normally own. More information can be found here. Click on explore free content to start browsing books.

Project MUSE - Participating publishers are making their content temporarily available for free through the Project MUSE platform. Content includes research in the humanities and social sciences, from university presses, scholarly societies, and related not-for-profit publishers.

The Boston Public Library or Minuteman Library Network both provide access to ebooks that can be downloaded. The Boston Public Library offers an e-card that could get you started right away if you meet their requirements.