I love E's used books idea best for my own stocking. You can't ever have too many in my world!
I also seem always short on and delighted over hats, mitts/gloves and socks because I run cold and hv Raynaud's. The year I got ear muffs i was jubilant! My hands, feet and lips always appreciate little tubes and pots and chunks of buttery things. And I adore sweet or spicy treats, bath supplies and practical little tools like sweater shavers, lint rollers, razor refills, hair accessories, bookmarks, letter openers, trinket boxes (I adore containers!). One of the sweetest stuffers I ever got was a case of CDs recorded with my favourite artistes... not albums but sets of favourites mixed together in petsonalised playlists.
For giving, I have a reputation as a giver of suds. Soaps for big people and bubble wands and bubble baths for little ones :-p
Pretty push pins and magnetic notepads for those who plan their lives up a wall are favourites. Seed packets for those with gardens. Thimbles or new needles or bobbins or a packet of buttons or a few yards of trim for those who sew or crochet. Earphones for music lovers. Stress balls as a loving hint to the cranky puss in your life. Claim vouchers for time spent with you, a favourite treat, a massage, a chore exemption or chore done etc always go into family and close mates' stockings.