The rules say that the spell can show "even another spell" and since you can learn or prepare spells in written form unless they're a scroll and nothing in the rules says anything different, you can learn and prepare a spell from a Secret Page. Now you either cite the rules how not every recording of a spell lets you do this or the discussion is over.
Fine.
By your rules I can use secret page to put any spell in a spellbook. Want
I Smell Your Fear? No Problem.
Mass Heal? Yawn.
Otyugh Swam? C'mon Give me something HARD! And since it's in my spellbook, I can study it and learn it and cast it.
Why limit myself to spells other people can cast? Why not just secret page up spells on the fly?
Wall of Starburst. Makes a wall of tasty, chewy, fruit flavored candy.
Oh wait... That would be bat-shit insane.
Since it clearly isn't the RAI to allow a wizard to use secret page to become the undisputed ruler of time and space. Maybe, JUST MAYBE, we should consider that there needs to be some limitations on how this "another spell" can be used. Since the process is... oh... I dunno... PUTTING A SPELL IN A SPELLBOOK... maybe we should also use the rules for putting spells in spellbooks? Huh? Dontcha tink?
It's one or the other. Either secret page can put any spell in there, or it can't. It doesn't say Arcane only. Doesn't say spells you know. Doesn't say Spells that EXIST! It's another spell. If I choose, I can make a secret page of a spell called, "
Tenser's Slipped Disk" When you cast it, your back gives out.
There is no reason WHY I cannot put this spell in the book. I am not disputing that you can put any spell you want in the book, BTW. What I am disputing is that a wizard can read that spell and memorize it and cast it.
So either you allow my Tenser's Slipped Disk spell, or you don't. If you don't, YOU are setting up an arbitrary limitation on the spell. My limitation is not arbitrary. It's based on the rules for scribing spells into spellbooks. In my opinion. All you need is a spellbook and a pile of gold. Bam. Gold disappears, spell in the book. The spell can be memorized. Secret page is transmutation. The need for ink or pens or crap is unimportant. Just the 100 gp/level cost. But it can't "cash in" a spell in a book and liquidate it for something else.
Well, you know what, I take that back. One page in a spellbook is worth 50 gp. So, if you want to permanently destroy two pages to get one new page in your book, I would allow that. But the other two pages of spells are gone, never to be used again.