There seem to be many different scarf joints, and I'm wondering which specific scarf joints work best for certain scenarios. Is there any literature or guidance for this?

The garage I'm building calls for ~23' long rafters (8x8) supported by purlin plates and eve plates. I'd like to use two timbers for one rafter and use a scarf joint over the purlin plate. Is there a certain scarf joint that would work best in this scenario?

The plates (both eve and purlin) are also very long and will require multiple scarf joints in each plate (8x10s). Seems that placement of these scarf joints is best over a knee brace rather than over a post. But once again, is there a certain scarf joint that would work best for plates?

Sometimes you need to use a scarf joint that works for how you're going to erect the frame.
If you're going to assemble the two halves of the rafter on the ground and then fly them up there as one by a crane.
Or are you going to fly one half up and then the second half.
There shouldn't be a lot of end to end tension in the rafter pair, so a scarf that could work would be a bridle scarf joint show here: https://timberframehq.com/timber-frame-bridle-scarf/
But that should be checked by a qualified, and experienced timber frame engineer, to be sure.
Jim Rogers

The best book on scarfs is one from England,I think it was Cecil Hewett's book. here: https://www.tfguild.org/store/english-historic-carpentry
I can't see my copy on my book shelf right now. I may have loaned it out to a friend.
Jim Rogers

Again doing a long building usually requires the plates to be flown up by crane to set down onto the top of posts and braces.
Thus the scarf should be one that can easily join together vertically.
The one I use is called a "halved and bridled scarf" as shown in drawing 19 in the scarf section (chapter six) of the Historical American joinery book. Here: https://www.tfguild.org/store/historic-american-timber-joinery
This is a good scarf joint and can be assembled up at the top of the posts.
Good luck with your project.
Jim Rogers

Thanks Jim, I appreciate the responses. I have the English Historic Carpentry book, so I will take a look at that. I will look into getting the other book. Seems like there's no go-to place for all things scarfs. I have been thinking about the raising and how each section will go up, so I will continue to do that for the scarf joints as well. I imagine the rafters to go up one at a time, since they will have the support of both the eve plate and the purlin plate. This should allow me to assemble the rafter scarf on the deck and then raise it. I read a post on TimberFrame HQ that showed rafter scarf located above the purlin plate (unlike a plate scarf which seems preferably located to one side of the post). Is that true for rafter scarfs?

One more question regarding scarf joints...
When designing the joint, are there rules of thumb for the dimensions? Such as length of bridled tenons, peg placement and size, tenon width, angles for undersquinting, etc?

Could be better at that location so that the lower half is bearing on two timbers.
Jim Rogers

The tenon on a bridled scarf should be long enough so that the relish doesn't blow out the end of the tenon. So you could use the 4 diameter rule. That is the hole location should be 4 diameters from the end of the tenon. Or consult the latest peg locations document put out by the timber framer engineering council on peg locations.
As for peg size. One standard is that the peg size is 1/2 the size of the tenon width. So for a two thick tenon a 1" peg should be used.
I don't recall any angle standards for undersquinting. But again as mentioned in another thread about scarfs you should make the overlap between to pieces four times the depth of the timber. 10" deep then 40" long scarf.
Jim Rogers

The angle would be determined by the depth of the undersquint. And by the depth of the timber. What I mean is it would be one angle for a 10" deep timber and a different angle for a 12" deep timber, if they have the same depth undersquint.
Jim Rogers

If you really can't get full length rafters (and 23' should be pretty easy), I'd go with something simple like a 12" half lap at the purlin plate.