don't know if it's cool to post another website's links but there are a number of sites that list 'drops' where you can search for mob that drops what you want/need.
The one i checked said summon mass poltergeist cubic was dropped by Gigant Raider or some such--not sure where it it though. Supposedly lvl 39.
I'm not sure if 'summon poltergeist cubic' is same as 'summon mass poltergeist cubic'.
if your class is PS i think that the spellbook she is referring Spark cubic and drops from the Hatu Windus (http://www.mylineage2.com/monster.php?k1=level&npc=933) its easy to get since the mobs are 1/2hp but is social to the panther and bear just pull it with your summon to a non social range and just kill it.
The sb is not for me, but for a clannie. He is a shillan knight (sorry about the misspelling) and had ask the clan for help in getting his sbs for his char. The three he ask for help with was Summon Vampiric Cubic, Summon Viper Cubic, and Summon Poltergeist Cubic. I have found the mobs that drop the spellbooks for the Vampiric and Viper Cubics, but I just can't seem to find the Poltergiest one. Ive looked of the 2 other sites that I know of that have lists of items, sbs, and such, but still no luck. Thanks for responding. I could use any help offered.