Elves, Wights, and Trolls by Kveldulf Gundarsson is pretty cheap to pick up used, and is such a fantastic resource that, in retrospect, I would have paid 3 times the price.
As the title says, this book is a study of all kinds of spirits the ancient Germanic peoples would have classified as 'other' - or not Gods. Gundarsson does a good job describing the fluidity that these categories had to the ancients that we, as modern peoples who like putting things in their little distinct boxes, don't often grasp right away. Honestly, on picking up this book, I was hoping to receive some clarification on what exactly a Jötunn was, compared to a land wight, compared to the alfar, compared to a troll or any number of other words the Germanic peoples had for these beings. There was not a lot of clarification, just an acknowledgement that the evidence seems to say that these categories were not at all distinct; and certainly none of them were definitively classified as 'good' or 'evil' to the Germanic peoples. Gundarsson does give some ideas as to what the distinctions may have meant in the past, but this UPG is pretty clearly marked out.
There is a separate chapter for each type of being, mostly devoted to folk stories of the spirits, and appearances and actions in the lore that we do have. Through these remnants, Gundarsson attempts to re-imagine what these beings may have been to the ancient Germanic peoples; and gives helpful suggestions on offerings, which spirits to propitiate with them, and which spirits to avoid. My personal favorite was the portion on house spirits, which not only contains several folk tales, but many cautionary mentions that folk practice seems to suggest are prudent. As someone who grew up in a home that still held some lingering beliefs about house spirits, it was very enlightening to see where some of these ideas and practices may have originated.
There are very interesting portions on the Jötnar, and Gundarsson is not hostile towards them in his writing. He even suggests that Thor fought the Jötnar, especially females, in the lore because the main foe of the ancient Germanic peoples was the harsh environment they lived in - but now that humanity has shown itself capable of permanently damaging the environment, perhaps the scales ought to tip from malice towards concern for the Jötnar.
The one concern I had was Gundarsson's tendency to ascribe equal importance to all his sources - whether archaeology, ancient lore, or very recently recorded folk tales. While interesting, there is certainly a vast amount of difference between the archaeology's conclusions about the culture of Germanic peoples around 0 BC and the culture of Northern Europe in the 19th century when many folk tales were recorded. Gundarsson does make a great effort to reference his information, which diminishes the problem greatly, but I feel it still effects his conclusions in a way that I personally don't prefer.
All in all, it is a fantastic book, and belongs on the bookshelf of any land-honoring Heathen. I also think it would be helpful to any Pagan looking to honor land spirits, even if they are not specifically interested in Germanic culture, because of its wealth of information - seriously, this book is just packed with info. I've read quite a few books on the fae of Celtic lore, and none I've found have been so helpful or informative as Gundarsson's is on the land spirits of Germanic lore.