For anyone who is interested in seidr, I would recommend Katie Gerrard's "Seidr: The Gate is Open". A few caveats, though:
1. Gerrard's practice appears to be heavily influenced by Diana Paxson's group Hrafnar, which seems to garner a mixed reception among Heathens. She does repeatedly state that at the time she started developing her practice, there weren't as many resources available to draw on, that said, it is something to keep in mind.
2. She repeatedly states that this is her way of doing things, based on what works for her. Her research appears sound and she does voice some interesting theories. Why, for instance, is the seeress in Greenland Saga given the heart of one of every animal on the farm? Does she just really like organ meat, or is there another reason?
I don't know if I'd recommend it to a complete beginner (Gerrard recommends that beginners to trancework pick up "Trance-Portation" by Paxson, which I also have) but I think the techniques are accessible enough that someone who is already familiar with trancework can start to use them in their own practice.
I know it's not really about Asatru or Heathenry in general, but it might be of interest to people who are interested in seidr specifically. There don't seem to be many "how to" guides out there.