The High Crusade is about as easy a read as you will likely come across. It is fast-paced to a fault, unchallenging, and fun. I believe I read the entire novel in two days (on workdays, too). The plot is simple: an alien ship, a precursor to invasion, lands in medevial England but is quickly captured by the local Englishmen and repurposed by them. I won't spoil what else happens, but you won't need to read very far to get into the story. A lot happens, even if it is the same thing happening over and over, and the reader is unencumbered by complex shades of good and evil or pesky characterization.
The Englishmen in the story are universally noble (the only conflicts among them are romantic) and adept in combat against far more technologically advanced foes. Longbowmen easily defeat enemy airplanes and medevial siege tactics make short work of tanks and artillery. Anderson gives just enough justification to allow suspension of disbelief, but the focus here is on adventure and fun.
Perhaps the strongest aspect of the novel is the style in which it is written. The story is narrated by an English monk who is the spiritual advisor to the noble leading the charge against the aliens, and therefore his narration uses medevial euphemisms for the alien technology. Additionally, the monk tries valiantly to fit new knowledge about space travel into his Christian worldview with mixed results. The writing is appropriately overdramatic and romantic, sort of like Ivanhoe on peyote.
Read this for a good time and quite a few smiles, but not for deep contemplation.
Be seeing you!