I was impressed with Poul Anderson's minor novel, Shield. The other reviewers have commented that the novel is dated. Yes. But so are almost all sci-fi novels written in the 1960s when it came to describing computers etc. Also, it has been suggested that the redundancy of the technology in the "kinetic force shield" somehow detracts from the story. I would argue that if anything Poul Anderson's novel creates the vibrant world that many of these other novels lack (i.e. Asimov's Space Rangers). In addition, unlike the Space Ranger series this is slightly more serious and might appeal to a slightly older audience.
That said, the basic plot is as follows: Koskinen, a naive young astronaut, returns from Mars and an extended séance with the planet's friendly natives (thankfully never fully explained -- the "mystery" adds some depth) with a kinetic force shield. Unknown to him, every faction of Earth wants to get their hands on it including his government whom he is VERY reluctant to disown. He falls in with a local warlord and his mob which inhabits a World War Three crater decked out with pill boxes and the beautiful Vivienne. A bomb is placed around his neck and he is forced to create more force shields. In order to avoid spoilers I will stop at this point.
Poul Anderson manages to create a very interesting world: the inhabited craters, the cities attempt to suppress these crime dens. However, the characters are one dimensional of course, the plot semi-predictable, and the technology is not that original. That said, this is a fun read!