1) Find the text
Good site to find out-of-copyright books is:
but just googling around can find books other places too.
You want to find the text somewhere where on the web where it isn't an image. Try selecting a part of the text with your mouse to confirm it isn't an image.
2) Grab the text
The book might be either just plain text, or html. Either one will work. With HTML you are getting more of the formatting of the original book, and it can include pictures. This can be either a benefit or a hindrance.
To grab the text, select it all in your web browser and copy and paste it into a blank document in your word processor.
3) Remove the hard end of line
You will almost always find that there are hard line ends corresponding to the end of lines in the original layout of the book. You will most likely want to remove those unless you are trying to make a perfect fascimile copy.
The easiest way to see this is to turn on the "show special characters" mode on your word processor so that you can see them. You can just delete these hard end of line characters by hand, but the lazy way to do it is:
You want to not lose the hard end of line that indicates the end of a paragraph. Luckily you can distinguish those because they are usually two end of line characters together (to create the blank line between the paragraphs).
So:
replace all pairs of end of line characters ('^p^p') with some special phrase ('$$$$$$').
get rid of all remaining end of line characters (by replacing them a single space).
replace the special phrase with a single end of line character.
4) Layout the book into signatures
The exact details of how to do this will vary with the word processor you are using. Microsoft Word supports a multi-signature format (called "Book fold") where you can specify how many sheets of paper there will be in each signature.
I think all the other basic word processors allow you to create a single signature "pamphlet". In that case, you will need to separate the text into individual documents, one for each signature. If you are doing that, you might want to do that after step 5.
5) Play with the formatting
Just a few things to remember: Pick your paper size based on what range of paper sizes your printer will accept. Make sure to leave margins for trimming. You also must have even margins to allow for duplex printing.
6) Printing your text block
You'll likely be doing "manual duplexing", which just means that you are flipping pages over to allow your printer to print on both sides. You'll have to figure out the best way to do this for your individual printer. The one trick I'd suggest is to print the book one signature at a time because if you ever misfeed a page or get somehow a bit confused, you will only have to reprint that one signature.
Also, I found my printer worked better if I added enough blank pages to the document to make a full final signature.