As a general rule, people associate the era of the third Doctor (Jon Pertwee) with stories that make political comments about the real world. This book is based on a serial that was first broadcast in 1964 and looks at the possible dangers of insecticides.
The TARDIS arrives in 1964 and, due to an accident on landing, the ship and its crew are miniaturised. Much like the later "Land of the Giants" TV series, their adventures tend to centre on the threat of full-sized items to miniaturised people. As you can imagine, the insecticide DN6 becomes a major peril.
Running parallel with this story is one for the full-sized people. DN6 is under investigation, and its inventor will stop at nothing to prevent his own financial ruin if it is banned. While the actions of the full-sized people have consequences for the Doctor, Susan, Barbara and Ian, it is not towards the end of the stories do our travellers start to have an impact on them.
The novelisation is a straightforward adaptation by the prolific Terrance Dicks. While no literary masterpiece, it is certainly readable. You may have to excuse some very questionable science, but that's not something new with Doctor Who.