Your fanatic probably already has a brick separator (There is a $5.95 one that is not as complete as the $8.95 one), but if she doesn’t, buy her at least two, as they are worth their weight in gold to any Lego fan. They also make a good gift to Lego fanatic parents, as well. For a more advanced Lego fan (older than 8 or so), Brick Journal (Vol. 1 or Vol. 2) is a wonderful magazine. I personally found The Unofficial Lego Builder’s Guide very helpful (but I am far from an expert).
For younger children(4-10, maybe?), Cool Cars and Trucks by Sean Kenney is a great read. It is worth watching the author’s videos on Amazon and looking at his site, as he has achieved the very rare “Lego Certified Professional” title (there are only 11 in the world!). I really loved The Lego Book from DK, and my son loves it, too. It’s a beautiful slip-covered book (more for grown-ups, but has tons of pictures) about the history of Lego, with a second book just about minifigures. Also, a nice photobook of your fanatic’s creations is a thoughtful gift, too.