I went to the bestiary and searched for the keyword soulbinder.
Click here for the search results. Yes, all you have to do is say "bind my soul" (may have to hail him first) and it costs nothing. Soulbinders were added during the time I quit and were a very welcome addition when I came back to EQ.
Skin like wood and other similar spells don't cause your HP to go down. They cause your max HP to go up, but you don't gain those HP until you regen them.
HP are listed as Current HP/Max HP. For example, if you have 100 HP and are at 100% health, your HP will read 100/100. If an orc hits you for 15 damage, it will drop to 85/100.
Now, assume skin like wood gives you 20 extra max HP (made that number up, too lazy to look it up). At 100% health before you cast, your HP will be 100/100. Immediately after you cast, your HP will be 100/120, which is 83% health. Since your health bar only shows your current percentage of max HP, it may appear that your HP go down upon casting, but it is really just that your max HP goes up.