If she's playing 5E and she's only interested in stuff that might help her as a player, she should also check out the Sword Coast Adventurer's Guide. SCAG is mostly about adventuring in the Forgotten Realms setting, which is the default setting for 5E, but it offers a batch of new class options and subclasses mostly tailored for Forgotten Realms. (Though they're fairly easy to adapt for non-FR settings.)
Volo's Guide to Monsters might be worth a look, too. It's mostly about the lore of various D&D monster races, and includes a bestiary featuring new monsters for players to fight against, but it also has a chapter with stats for new playable races, including the aasimar and the goliath.
Xanathar's Guide to Everything comes out later this month and is probably going to be the best resource for new player options. It's going to include around twenty-five new subclasses, new spells, new racial feats, and a system to give characters a randomized backstory.
There was also a digital-only companion to Elemental Evil, a 5E campaign produced by WotC. It's called simply the Elemental Evil Player's Companion. This isn't actually something that needs to be bought, it can be downloaded legally and for free at Wizards of the Coast's website. It includes stats for new playable races like the aarakocra, deep gnome, genasi, and (again) goliath. And it has a bunch of new spells that have an elemental flavor to them.