I'm planning on taking ritual caster at 8th level and was wondering about the rules around putting this automatically known aberrant mind spell Rary's Telepathic Bond into the ritual book. (the link doesn't show it as a ritual but the spell is listed as such) 

Ritual Caster:

You have learned a number of spells that you can cast as rituals. These spells are written in a ritual book, which you must have in hand while casting one of them.

• When you choose this feat, you acquire a ritual book holding two 1st-level spells of your choice. Choose one of the following classes: bard, cleric, druid, sorcerer, warlock, or wizard. You must choose your spells from that class's spell list, and the spells you choose must have the ritual tag. The class you choose also determines your spellcasting ability for these spells: Charisma for bard, sorcerer, or warlock; Wisdom for cleric or druid; or Intelligence for wizard.
• If you come across a spell in written form, such as a magical spell scroll or a wizard's spellbook, you might be able to add it to your ritual book. The spell must be on the spell list for the class you chose, the spell's level can be no higher than half your level (rounded up), and it must have the ritual tag. The process of copying the spell into your ritual book takes 2 hours per level of the spell, and costs 50 gp per level. The cost represents material components you expend as you experiment with the spell to master it, as well as the fine inks you need to record it.

The second point seems to say I'd need to have the spell in a written form, not simply as one of the known spells on the psionic spells list.

But any class can create a scroll, so according to my reading of RAW I'd need to create a scroll first and only then could I record it in the ritual book? That seems kind of weird, why couldn't I just write it down in the ritual book directly? Or is there some way of doing this that I'm missing?

Xanathar's has guidelines for scribing scrolls; note that you need to either Know the spell or have it Prepared, and also have Arcana proficiency. Then you can copy it into your spellbook.

If you want to streamline the process, talk to your DM about what they'd allow you to do differently for copying Known spells into the ritual book, just realize that this isn't part of RAW.

The way I was able to understand it, having the spell in easily discernible written format (a scroll), helps with the process of understanding it as a ritual. That kind of casting  is a whole other kettle of fish than regular casting, the latter is forcing your Will into reality through individual resource expense. (Hence why Sorcerers cannot cast rituals, because all of their magic is achieved through raw personal will.)

Ritual casting however, is more like coaxing it into reality on its own form. (Like when Hammond was coaxing the baby raptor out of its shell in Jurassic Park, or how Costner’s character coaxed Shoeless Joe Jackson’s ghost back to our realm by building a baseball Diamond in the middle of his cornfield.)

But, any spellcaster can cast from a scroll if that spark is natuive to their Class spell list. So they are stripped down enough that even a Sorcerer can cast o from a scroll. By taking a spell you could cast through sheer chutzpah and figuring out the common denominator by first creating the scroll, than it infinitely easier to translate that into a ritual casting for your Book of ancient secrets. So the additional cost to first scribe the scroll is the price of the extra experimenting a PC has to do.

Without a scroll formatted

Quote from naruhoodie >>

Xanathar's has guidelines for scribing scrolls; note that you need to either Know the spell or have it Prepared, and also have Arcana proficiency. Then you can copy it into your spellbook.

If you want to streamline the process, talk to your DM about what they'd allow you to do differently for copying Known spells into the ritual book, just realize that this isn't part of RAW.

Hello Naruhoodie,

I do not know if I agree with this call. The feat gives you everything you need for the feat to apply fully. If he were trying to make a scroll from what is found in his ritual book, then I believe the criteria for scribing scrolls would apply. The resources to copy are different as well. In the feat, copying is 2 hours per level plus 50 gold for each level. While scribing the scroll requires significantly more time per spell level and significantly more gold per spell level after the first. Additionally, you need the full spell components to imbue the scroll while you do not for the ritual feat. To me, this suggests that these are very different processes. Scribing a scroll is basically creating a hold action spell that any caster can use. While the ritual feat is copying a spell from one magical source into another.

Really the only problem here that I can see is that the spell he wants is too high for him while his character is only at level 8. The ritual feat only allows the copying of spells that are half the PC's level rounded up. He would need to be at least level 9 to copy the spell to his ritual book. 

What the OP was wondering about is how to copy a Known spell into a ritual book, once they reach level 9 and gain Rary's Telepathic Bond as a known spell from their Aberrant Mind Sorcerer subclass. That's a different matter from scribing a spell scroll for a spell that's already in their ritual book, which is what you're talking about.

Edit: Since the Sorcerer class doesn't have ritual casting, it makes sense to me that they'd want to copy the spell into the ritual book, and then swap out the Aberrant Mind spell for a different 5th-level Sorcerer spell once they reach level 10.

Ah, my mistake. I had missed this very important context. Additionally, to support your earlier comment, Jeremy Crawford does state that for ritual caster to work, one needs a written version of the spell. So, for the ritual feat to work, he would have to scribe it to a scroll before putting it in the ritual book. You were correct and I misunderstood.