Maybe it's the language that is confusing you. Here is some text from Kreith's book:
The process of transferring heat by convection when the fluid flow is driven by an applied pressure gradient is referred to as forced convection. When this flow is confined in a tube or a duct of any arbitrary geometrical cross section, the growth and development of boundary layers are also confined. In such flows, the hydraulic diameter of the duct, rather than its length, is the characteristic length for scaling the boundary layer as well as for dimensionless representation of flow-friction loss and the heat transfer coefficient. Convective heat transfer inside tubes and ducts is encountered in numerous applications where heat exchangers, made up of circular tubes as well as a variety of non-circular cross-sectional geometries, are employed.
So this paragraph has four sentences, what do they say?
1. Definition of forced convection.
2. Boundary layers are confined in a tube or duct.
3. Hydraulic diameter is the characteristic length.
4. This part of the theory is important for heat exchangers.
Right? It's simple but hidden with rather terse language. Engineering books tend to be like that, using more words than necessary, and sometimes that makes things less clear.
Also you need to have the prerequisite knowledge. For example, we can already see that knowing what the characteristic length is and what it is used for is going to be important for this chapter. We can surmise that if we know that, what job the characteristic length plays, this chapter is not going to be difficult because it'll use it in a new way but it will be similar or analogous.
But we can also say that if we don't know what it is or don't know how it is used, this chapter will be more difficult. Probably they aren't going to explain it a lot and will just show what changes in this case. Perhaps this is what you mean when you say they discuss the same theorem with different words but it still isn't clear. It could be that you missed something earlier or don't realize that they are assuming you know something or are making a comparison or analogy.
Because analogy is very, very important. It is always important to ask, how does this compare to other situations that are similar? How is it the same and how is it different? Does it make sense why it is different like this and why it is the same like that?
For example, why does the author say "the hydraulic diameter, rather than the duct length, is used as the characteristic length"? Why should the diameter be used as the length? Until you know why, it is going to seem odd and there must be a reason why, and when you know it it'll make that much more sense.
So that's my advice, try to see what is important in the words, try to see what is assumed, try to see what analogies are being drawn, what comparisons are being made, and try to answer questions like the one above, about anything that seems odd. There must be a reason and it'll seem odd until you know the reason.