The first items to buy are definitely fashion or beauty related books because these look great in flatlays. My personal favourites are #girlboss, Capture Your Style, Love Style Life and Pretty Honest but Urban Outfitters and ASOS have a ton that you could choose from.
If books aren't the kind of thing they'd like then blog props definitely are. We love a flatlay so anything that would look great in one is a winner. I love browsing Urban Outfitters or other lifestyle brands for this kind of thing because they have a great range of things which could work.
Equipment. Whether its a fancy lens, tripod, ring light or even a camera strap - bloggers need their equipment to get the perfect shot so depending on your budget, this could be a perfect gift. Or if you've got a bigger budget then the Olympus Pen EPL-8 is always perfect!
Finally, fashion and beauty. These are what we tend to get a lot of but bloggers tend to post wishlists a lot so its a great source of inspiration for items, gift cards or the investment piece they've always wanted like a trench coat from Aquascutum. Beauty is a great present too and I've got a whole gift guide for that so its worth a browse!