Download the Bookscouter app for iPhone/android. You can use the camera to scan the ISBN code, or type it in manually (I used the Siri voice app to read it in). This searches hundreds of book buying sites for the best price. Every site has different prices - some books are a nickle on one site and $5-10 on another. Sort them into piles by site, then box them up, print out the mailing label, and ship for free. Any old university text books you have seem to give the best return. SF books, you might get a nickle to a quarter a piece.
Any books left over, take those to the library. They will give you a receipt, but it won't tell you how much it's for. You have to estimate that. If the value is over $500, you'll need a professional appraisal to support the deduction, which is too much of a hassle. So whatever you claim, make sure it's under $500. If memory serves, all these requirements are on 1040 Schedule A or some related, downstream form.
I just got rid of my 40 year collection of books last year when I sold my house, and this is how I did it. I made a couple hundred dollars, but over half were worthless and I had to donate them. My library sells them once a year in a big book sale, so it went to a good cause. I estimated at 50 cents a book for softcover and $1 for hardcover. I had over $500, so I put a few boxes in storage for donation in 2016.