ANSWERS!!
i hope i dont get into trouble here ...
In Magic of Faerune the following is printed:
Cover
Leather - 1lb - Hardness 2 - HP 0 - 5 gp
Wood, thin - 1lb - Hardness 3 - HP 1 - 20 gp
Metal, Soft - 5lb - Hardness 5 - HP 4 - 100 gp
Metal, hard - 5lb - Hardness 7 - HP 5 - 200 gp
Dragonhide - 2lb - Hardness 4 - HP 2 - 200 gp
Slipcase - 1lb - Hardness 1 - HP 1 - +20 gp
It also mentions the different types of pages, about Craft (Bookbinding). If you have Craft Wonderous Item, you can enchant your spellbook. One such is ...
Resistant - The book has a resistance of 5 against acid, cold, electricity and fire attacks (If you wanted you could do 10 to acid and forget about the others). Caster 6th level, Prerequisite Craft Wonderous Item, Endure Elements. Market Price +1000 gp.
I say if its such a big deal, give him Resistance of 5 against Acid. If he wants more, he can enchant it. Maybe give him some more Hardness (+1 or +2 maybe)
Hope this helps