Do you remove the paper and replace it with some other thing, or do you glue big sections of the book together, and if so, how? Do you not remove any pages and just do a collage or whatever on every page...
You can remove pages and you can glue pages together. It really depends on what you are going to do with them. For example, if you are going to add embellishments, then it helps to remove some of the pages to accommodate the thickness added by the embellishments. You can glue pages together if you need the page you are altering to have more to have more strength, maybe because you want to paint it or whatever. I've glued pages together when I wanted to cut a niche into the book; gluing together a number of pages creates the depth you need for the niche.
I know this is kind of old fashioned, but you might try going to the public library and checking out a few books that will teach you basic techniques, like Altered Books Workshop
by Bev Brazelton. Also, public libraries often sell old books as a fundraiser that can be used for altering. Hard cover books older books really work the best for altering.
I don't think you've ruined any books--you've simply created some practice books. You need to practice these techniques, try things out, so these practice books are part of your learning curve.