I'd like to make an altered book but I don't know the best kind of book to get. Or where to get them.
Depending on what you have in mind you can always start with a child's board book. These don't have a lot of pages but can stand up to various techniques without warping. You should sand the book down if it has a really shiny finish on the pages - that way your paint and glues will adhere. If you are looking for a 'regular' book try to find one that is sewn rather than glued. This can be hard to tell sometimes - open the book up and look at the spine. If the pages are right up against the spine it is probably glued. If it arches away it might be sewn - look at the pages and see if you can see any sewing. A sewn book will stand up to the altering process better than a glued one will. As to where to find them - used book stores, garage sales, even the library sometimes has old books for sale. Look for good quality paper too -some of the Reader's Digest type books can be printed on flimsy paper. Older,vintage, books generally have stronger paper than some of the newer ones.
Hope this helps!