Lending books

I often lend people books. In the past I used to be quite possessive about them, and hope to get them back, whereas now I generally resign myself to the high probability that I'll never see the book again (especially if I've lent it to a member of a religious order :P).

Increasingly now, books come in electronic format and I've been wondering about the legitimacy of lending people ebooks. Can I lend someone an ebook in the same way that I lend them a printed volume? Should I delete the file from my system while they have it? Am I more directly depriving the author of an ebook of legitimate income by making their book available to a friend?

A couple of recent examples: I have an excellent ebook that I've recommended and shared with several people. The author points out that the income from the sales of the ebook helps to support a single-income Catholic family with a disabled child. Should I encourage my friends to buy the book themselves to support her, rather than sharing it with them for free?

Another ebook was made available for free by the author when it was first published. Now, however, there is a charge to download it. Does this make a difference to the ethics of sharing the book?

Hmmm, perhaps not the weightiest of ethical dilemmas! Still, I would be interested in what people think...