The intercloud medium is an extensive, low density, very hot gas, probably carved out and heated by supernova explosions. They are relatively small and low density, and they are the source of the 21-cm radiation discussed in section 18.4. Their temperature is 100K. The other half of the mass is in the molecular clouds, mostly discussed in section 18.5. These clouds come in various sizes. The Rho Ophiuchi cloud in Figure 18.12 is a Bok globule, not an HI cloud.
Molecular clouds have various temperatures.
That's why the text gives the answer of 10-20K for a question about the coldest dark clouds, and why section 18.5 first gives 20K at the beginning, and 50K near the end. Note that molecular cloud complexes were first believed to be single giant clouds, and so they are also called Giant Molecular Clouds. Only a tiny fraction of the interstellar medium is in them. They are created by O and B stars. These stars are very important because they dominate the luminosity of a galaxy like our own, even though they are rare.