The FRCS is an outstanding book, highly recommended. The player's guide is also considered a core book by wizards, the new books refer to stuff in it (but you might get by without). It also explains the new cosmology further and incorporates several core D&D books into the Realms (Book of Vile Darkness/Book of Exalted Deeds, Expanded Psionics Handbook, Epic-Level Handbook) Those two make a good basis.
Now that you have covered the bases about what's new, you might want to know what's old. Real old. Get Lost Empires, it's great reading.
Lords of Darkness is another nice one - 28 or so evil organizations explained, 7 or so in greater detail. Everything from Drow over the major evil Churches to Zhentarim, Red Wizards, Shadow Thieves, Night Masks, Fire Knives, you name it. Twisted Rune, Knights of the Shield, and stuff like the Eldreth Veluuthra (fanatical elves out to eradicate humankind)
Other than that, I can recommend every single FR 3.5 book out there, with one exception: Champions of Ruin. Out of a line of awesome books this one is merely "okay". One of my main problems with is the power level of the legendary Elder Evils (Kezef the Chaos Hound, Dendar the Night Serpent, Ityak-Ortheel the Elf Eater), who have the reputation to be a threat to even the gods. They have CR 21, 26, 22 respectively. This is a joke. So Mask is running away from something that only hits him on a natural 20, but which he could take apart in less then a minute. Yeah right.
Rich Baker said that they wanted those beasts to be beatable by player characters, which are supposed to be the hot




, but that really doesn't explain creatures like Elminster (CR 39 with about 30 levels wizard and the chosen of mystra bonus), Ioulaum (Which is an elder brain lich wizard 40 or something like that, cr well past 50), or even Aumvor the Undying, a lich with effective wizard level 32
from the very same book. "Sure, we wanted the PC's to be able to kill Kezef as a small side-encounter, but then we inteded them to be pressed through a fine mesh by that Aumvor character."