Readers are a special brand of person, and it can be hard to shop for someone who has a million books on their shelves already (or a million stacks of books everywhere you turn). There's such a high chance that you'll buy them something they already have, and that's daunting. So here I am, an avid reader for as long as I can remember, to guide you in how to satisfy the readers in your life.
1. Highlighters
Sometimes, there are just great lines that you want don't want to lose and don't have a spot to write them down, so good highlighters are a great thing for a reader to have on hand.
This is also an especially good gift for college students and book bloggers, seeing that readers can color code the sections they highlight to make exam review or collecting thoughts before a book review easier.
2. Book tabs and sticky notes
This item goes right along with the last one, but it's more for the readers who love keeping their books pristine. This way, they can simply mark (with color coding still being possible) their pages with sticky bits that can later be removed rather than being permanently marked. Just like highlighting, these can be great note keeping tools, especially if it's in textbooks that they may sell in the future.
3. Bookmarks 
I know way too many readers who can never find a bookmark and will use anything they can find to keep from losing their place: notecards, receipts, paperclips, pens, who knows what else. Bookmarks can be found in any bookstore, and Amazon always has them cheap.
You can even pick your reader a bookmark (or a pack of them) based off of their likes to make the gift feel more personal. And if you're a bit artsy, bookmarks can be made with materials as simple as index cards and washi tape.
4. Gift cards to get more books
Thriftbooks (one of the best sites ever for finding cheap books in great condition) and smaller bookshops often have gift cards available, so you can even give your reader an excuse to go their favorite bookshop for the holidays.
5. Mugs
Regardless of preference, it can be said that there are fewer things nicer for a reader than curling up with a good book and their warm drink of choice. Mugs can make any reading experience feel a little homier, and they can even be used without actually reading, so it's a double win. Even better, you can even find bookish mugs on websites like Redbubble. A variant of this for your friend who's always on the go would be travel mugs.
6. A container of their drink of choice
Like I said, there are few things nicer for a reader than curling up with a good book and their warm drink of choice. So, buy them that drink of choice. Besides, it goes well with a new mug if you get them one. If not, you're still helping them on the road to relaxation by resupplying them. If you don't know what they like well enough to get them a container of it, you can always get them a gift card to a coffee shop or something similar.
7. Copies of books you love
There's no easier way to connect with a reader than through words. Gifting them with a book you love not only gives the two of you something to discuss but gives them a better sense of who you are and what matters to you. Besides, you never know if you'll introduce your reader to a new world, character, or world that they'll fall in love with.
8. A reading tracker
This was actually the gift that gave me the idea to start this list. These are incredibly useful for book bloggers but will be appreciated by just about any dedicated reader.
A variety of these can be found in bookstores and places like Amazon, and they do exactly what you think they would: help readers keep track of the books that they've read and what they thought of them. I personally own and love “The Book Lover's Journal: My Personal Reading Record" by Rene J. Smith.
9. A planner
This item goes along with the previous one. There are a lot of avid readers out there who like to set a goal of how many books they'll read during the year, and having a planner can help them keep track of their progress as the year goes on, because 365 goes by much faster than expected.
This is also great for a reader who happens to be a student for tracking course readings, homework, and deadlines. Even non-students can use these to keep track of bills, appointments, and things. This is a gift that you can't really go wrong with.
10. A copy of a classic
For a person who loves words and fictional worlds as much as most of us do, it can be so easy to get caught up in a particular genre or time period. A good way out of this is to give a reader a copy of a classic.
Not only does this get them out of their comfort zone, but it's eye-opening to see the work of a writer that may have inspired one of their favorite writers, but certainly influenced the world of writing at the time it was published and beyond.
11. A film adaptation of a book (or vice versa)
You can make a day out of this one. Film adaptations never quite live up to the book, and this can range to “eh" to downright terrible. So get your reader a film adaptation of a book, get some popcorn, and make an event of it. Watch the movie together, and then discuss. If worse comes to worse, you can always make fun of it together.
