I'm pretty sure they are lying, at least as far as that explaining the high prices. Here is why I believe this:
These are Apostol's excellent two volume undergraduate calculus textbook. Volume I is $231.25, and volume II is $189.49.
They were around $22 each when I bought my copies in 1977. I'm sure that NONE of the subsequent price increase is due to revisions and updates, because there have been no revisions or updates. Volume I is still the second edition (from 1967) and volume II is also still the second edition (from 1969). (There has been no need to revise or update them).
Based on inflation, these books should be about $80-90 per volume now (if we assume that $22 in 1977 was a reasonable price).
Interpreting that as a request to name the textbooks I find useful, I'll do that here.
The Gelfand Correspondence Program series
Basic Mathematics by Serge Lang
The Art of Problem Solving expanded series
When a student has those materials well in hand, it is time to work on AMC and Olympiad style problem solving,
and also the best calculus textbooks, such as those by Spivak or Apostol.
By far the best initial reading text is
Let's Read: A Linguistic Approach
but there are many other good reading series, including
Teach Your Child to Read in Ten Minutes a Day
(I devote more time than that to reading instruction, typically, because I use multiple materials)
and quite a few others. There is more junk than good stuff among elementary reading materials, alas.