Springer released a bunch of its e-Books for free, as one of those gestures of corporate-brand goodwill for the pandemic. The free-download period is slated to end the 31st of July. So if you have any interest in academic books, take a little while and read over the options.
Harish Narayanan kindly improved on Springer’s organization scheme by grouping them by subject and ordering them by title. Also adding their Goodread ratings, where available. You can find that grouping here, in a table easy to search. The categories are not perfect — Roman Kossak’s Mathematical Logic is grouped under “Religion and Philosophy”, for example — but it’s a good starting point. And all it’ll cost is download time.
I do not know whether this is region-limited, but if it is, it is limited in the most annoying and foolish way possible.