I've read in my English grammar that the word "most" (not "the") can mean very-very. For example " a most expensive book" means a very expensive book. If the noun is in singular, there is no problem distinguishing it from the superlative "the most expensive book", but if the noun is in plural, does it work the same way? For example, The books I bought yesterday are most expensive (meaning very). I am a bit confused.

Also, can you explain the following grammar example to me?

Being rich isn't ..... important to me.

a. much b. that c. such d. so much

According to my book, the correct is b, but I can't see why a and d are not correct.

Thank you for your time!
Most meaning very sounds sophisticated to me. I also associate it with British usage (but I may be wrong here).
I think someone else may be able to answer your question in grammatical terms; for me, that is the only one that sounds right here.

"Most kind of you !" , "I'm most beholden to you, indeed ! " You may hear that in a Victorian play .
These are expressions where "most" is used to mean "very " , "much" or "extremely" .

Being rich isn't ..... important to me. Depending on the context, "that" is used here as an intensifier to mean " not as... " or " not very" as in :

-Tom will give you back your moeny .
-Of course he won't ! I don't think he's that good.