Well, most probably many has the same problem, but there is no real cure because the spell book is filled by other spells as the many of the mods fill it full. Now the only known medicine for this is the ShadowKeeper but it might give you errors(as it's a little old), or other modifing tools that can open your save folder, which gives you the ability to edit your chars etc. I might recommend NI(Near Infinity), but you might get lost to it so...
Thanks JM, I understand now.
There're only 24 slots for spell picks at each level, but there're 27 or so level 4 spells, so only the first 24 (in alphabetical order) got displayed. So, spiderspawn ("SP...") got displayed but stoneskin not ("ST...").
So I have found a quick and dirty workaround for that. First I pick 2 shitty lvl 4 spells that my sorc most likely will never use - in this case, Beltyn's Burning Blood and Iceshard. Using DLTCEP, I open that spell and change the Spell Name Description string from 79018 (Beltyn...) to 12057 (Web), does the same with Iceshard. Now these two spells starts with "W.." mean they'll will be pushed out of the list and enough place for Stoneskin and Spiritarmor to go back in. Not beautiful I know, but whatever floats your boat...
Now I'm doing a research to see if there're any other important spells being absence like that.
Cheers