Set aside two hours at a time when starting the downsizing of your book collections.
Prepare for the project by assembling sturdy boxes of manageable size because books are heavy! You may also want to have some plastic storage bins for those books you are reluctant to part with and want to reconsider later.
Start a book jounal with the titles and authors' names of books you may want to read again some day. If you decide to, you can check at the library or buy a paperback!
Call area used book stores to see if they are interested in purchasing used books, taking donations, and picking them up. Libraries have used book sales periodically and accept donations year round.
Think about whether you are still interested in reading about topics such as gardening, astrology, child rearing, or self help books. Let the textbooks go! Textbooks are updated regularly, and practically any information you need can be accessed on Google or You Tube!
Rather than collect old magazines, save copies of articles in a "cloud" and recycle any magazines that are over two months old.
There are some books you just can't part with, such as your collection of Winnie the Pooh that you are saving to read to your grandchildren or great grandchildren, photo albums, and yearbooks. Even when my mother had dementia, looking at her old photo albums piqued her interest, and I was glad we had kept them!