Which book to alter? Do you have some old books you no longer want? Even children's books will do. Select a book the size of the images you have collected. In one of my books I used images of fashion models so the book had to be tall enough for their long bodies. If you don't have any old books around, go shopping for one.
Where to shop? Garage sales. Thrift stores. A local thrift store has all hard backed books marked at 69 cents - regardless of age, titles or size. Our town has a fabulous Friends of the Library Used Book Store and it's less than a mile from here. I visit it often. They have no idea what I do to their old books!!! And, by the way, unless you have worked in a library, you can't imagine how many books end up in landfill every year. Libraries get donations of huge numbers of books that do not fit their collection policy and won't fit on their shelves, or are outdated. Libraries have nothing to do but filter them out to other libraries, shelters, prisons, sellers, or the dumpster.
Look at the paper inside the book. If it is very brittle and yellowed or stained brown - it is probably not a good candidate for altering. Brittle pages tend to chip away when you try to cut them. If the book is really old, make sure it is not moldy. Smell the book. Some people are sensitive to some papers and inks. I'm very sensitive to newsprint and many of the old books - particularly the brittle pages. I mention this because it is something people usually don't think about - but you will be working very closely with your book and it needs to be health friendly. Get one that won't make you sneeze!
Check the binding. Is it secure? If it isn't, pass it by. You can find another one that is. Make sure the pages are sewn in rather than glued in. When you are altering the book it puts a lot of stress on the binding, and the glued pages will often come loose or whole sections may fall out.
Refer to Lesson 3 on Themes - if you want to tie the theme of your altered book to the book title.