According to a book that I’m reading (which does not have any obvious inaccuracies) radio reception is better after sunset. Is this true? If so, why?
See here for details. Basically, it has to do with the ionization of the ionosphere. 
I guess less thelaron radiation when the sun is not in line-of-sight
nasty stuff, that thelaron radiation
In addition to the natural ionization effect, the FCC helps out with allocation of frequencies. Some stations, especially in smaller markets, are only licensed to broadcast from sunrise to sunset, thus reducing the interference on other stations on the same frequency or other nearby frequencies. In other words, if you have an AM station at 950 kHz in your town, and another station miles away is also broadcasting at 950 kHz you may get some interference. But if that station shuts down after sunset, that interference goes away.
There are a handful of powerful AM stations around the country designated by the FCC as “clear channel” stations, meaning that no other station in the entire country shares their frequency, and they are licensed to transmit at 50,000 watts, the most power allowed. At night, it is possible to hear these stations from many states away.
On preview, I should make it clear that both the ionization effect and the “clear channel” principle apply only to AM stations and not to FM. Because of the difference in the way that the waves are propogated, FM is not as severely affected by ionization.
Or, you could take a look at these threads:
radio signals at nite…
Strange Radio Reception
Let’s be clear about this. The propagation effects under discussion are dependant upon wavelength. Longer radio waves propagate farther and can bounce off atmospheric layers in ways that shorter wavelengths cannot. In this case AM and FM mean the broadcast AM and FM bands, not the type of modulation.