Sure! Due to recent computer trouble, I've lost my stash of bookmarks, so I'm having to take a scattergun approach and haven't read or checked a bunch of these, so caveat emptor. They may not be directly what you want, but they may help you get there.

Kurai's stuff:
* List of soulbonders (might have links to jump off from): https://web.archive.org/web/20001207213700/http://kurai.com:80/sb/list.html
* Main SB page: https://web.archive.org/web/20010606173230/http://kurai.com/sb/main.html
* Soulbonder Essays (can't vouch for quality or relevance): https://web.archive.org/web/20020628193305/http://kurai.com/sb/essay.html

Musesong's post about it: https://musesong.insanejournal.com/1968.html

List of communities and users who list "soulbonding" as an interest:
* InsaneJournal: https://www.insanejournal.com/interests.bml?int=soulbonding
* Livejournal: https://www.livejournal.com/blogs/en/soulbonding
* Deadjournal: https://www.deadjournal.com/interests.bml?int=soulbonding

It's badly gutted, due to being locked down in 2008 and deleted in 2018 but you might find something useful in the archives of the LiveJournal soulbonding community: https://web.archive.org/web/20220000000000*/http://community.livejournal.com/soulbonding

Seiryuu's stuff:
* https://web.archive.org/web/20060519141130/http://www.seiryuu.org/~raincrystal/soulbonding/ (This post links to the Bronte Sisters, Edith Warton, and so on interacting with their characters)
* Links: https://web.archive.org/web/20060519141130/http://www.seiryuu.org/~raincrystal/soulbonding/

Ecthros's pages about it:
* Home page: https://web.archive.org/web/20030604055052/http://echthros.net/others.php
* How I Soulbond: https://web.archive.org/web/20030606131027/http://echthros.net/others.php?page=how

A bunch of the old stuff I have is from Eclective, which can definitely range into the spiritual:

* The archive of their old Child of Mana soulbonding pages: https://web.archive.org/web/20010625232812/http://childofmana.tripod.com/sd3main.htm
* Their old SB FAQ isn't linked on that index, so the direct link is here: https://web.archive.org/web/20011226155411/http://childofmana.tripod.com/soulbondingfaq.htm
* Their What Soulbonding Is Not (ableism warning): https://web.archive.org/web/20011226155411/http://childofmana.tripod.com/soulbonding_what-it-isnt.htm
* Soulbonding Spirituality: https://web.archive.org/web/20011226154359/http://childofmana.tripod.com/soulbonding_spirituality.htm
* Do You Believe In Soulbonding at First Sight?: https://web.archive.org/web/20020222234924/http://childofmana.tripod.com/sbafs2.htm
* Multiplicity, Midcontinuum, and Soulbonding: https://web.archive.org/web/20020918063730/http://childofmana.tripod.com/multiplemidcont-intro.htm
* Archive of their later 2003 SB site, may have a lot of the same, but also possibly new stuff: https://web.archive.org/web/20050112192159/http://www.yonicmafia.com/soulbond/
* Archive of their even later 2004 SB site, same caveats: https://web.archive.org/web/20040503180101/http://soulbonding.tripod.com:80/soulbonding_glossary.htm

Haven't read it, but Laura Gilkey's The Trinity might be relevant: https://www.fictionpress.com/s/735472/1/The-Trinity

Since the term "soulbonding" wasn't coined until the '90s, it's a different thing if you're looking specifically for the phenomenon of people talking to their characters (and not necessarily having an identity or community for it). I could dig around for that, but most of what I'd have would be media ABOUT that phenomenon (the movie "Stranger than Fiction," for instance).