Is it advisable to buy downloadable textbooks, and if so, what's a good service for this?

I’ve heard that there are sites where you can pay either a set yearly fee (ex. $150 a year), or per book, and download your textbooks to your computer. This sounds awesome, but I’m afraid that I’d get a service like this and then have half my books not be there anyway.

If it matters, I’m a third year psychology student.

Gracias.