Just wonder if there are any equipment and/or tomes or something that will raise the user's charisma (permanently or when equiped), and will the increase in charisma help the sorcerer's casting ability? Thanks.

I found several rings that raise charisma by one or 2 points when worn.

so does the increase of charisma affects sorcerer's ability to cast spells? If yes, what if, say, I learned the 9th level of spell with a CHR17 and +2 from a ring, then removed the ring?

try make oswald to make for you items from wolf skin (during chapter 3 in crashed ship, earlier there wasnt this option), for example cloak, gloves etc. each item increases your charisma and cold resistance by 1 point, but when i tried use two items made from wolf skin my charisma still was increased only by one point :( (
bug or maybe this was planned?
this is really quite a cheap way

bonuses of exactly the same sort don't stack. So two +1 Cha items still only give you +1 Cha. A ring of protection +2 (Deflection bonus to AC) and the Robe of Enfusion (+2 Deflection bonus to AC) don't combine, you only get +2.

Different types of bonuses do stack, though. So you can have armor bonus, deflection bonus, dodge bonus, generic bonus, natural armor bonus, etc to your AC. Attributes, however, don't have all these different options.

Oh, and yes your equipment based attribute modifiers should help you to cast spells.

In NWN, stat stack... in P&P too I think, it's not treat exactly like armor bonus

I'm pretty sure they don't stack in p&p, but then there are a lot less +stat items in p&p play generally so its kind of moot.

IIRC in NvN you can wear two diffrent pieces of equipment with stat increase and they stack together.
But AC types in NvN dont stack.

It seems NvN only went for some rules and not all :(

So what will happen if I learn lv9 spells with a charisma enhancing equipment then took it off? you'll lose the casting slots or what?

You will not loose the casting slot, but you wont be able to cast spells of higher level than 7. (Given that your charisma is 17.) Higher level spells will be greyed out.