Some of the spells in the game, eg: Flesh Ward, have a number at the bottom of the card. I couldn't find anywhere in the rulebook or reference guide that explains these numbers. What do these numbers mean? And if somebody could please point me to the section in the rulebook that explains these, that'd be great.

Some of the spells? Check again, if all spells have this number then it's for reference - so you can say for example "Hey what's the deal with Flesh Ward version 3?" - useful when you need an official clarification.

If indeed only some of the spells have this number, then I'll need to check on my game copy.

And the reason why they have the numbers, as with conditions, is that the backs are different. So when you first draw a spell or condition, you won't know quite what the consequences may be.

Yup, all spells have numbers for the exact reasons specified in this thread: having a card identifier (useful also when you check the components, to be sure you have all you need). In the same way, encounters have progressive ID numbers, and so on

Wow, I hadn't looked deep enough to notice all that. Thanks erbody!

I never noticed this... they're on the back right? I hope so.

Yes, they are on the back, so that you can't use your memory to know what will happen when you flip the card

Don't let these people fool you. The numbers on the back form a mathematical algorithm that, when correlated with certain ancient alphabets, compose the words needed to complete an elder sign.