Best eBook website for PhD students
If you don't have qualms about downloading from a site that has been convicted for mass copyright infringement, then Library Genesis is probably the largest one. It is basically Scihub's sister site for books.
Their Wikipedia page contains up-to-date links to access it (which may be blocked by some internet providers).
Check the legality of what you download. For instance, where I live you risk a 154€ fine for illicit downloads for personal use, without other aggravating factors. (I think at least -- IANAL.)