Very briefly, so as not to pre-empt your pleasure of reading this beautiful and easy to understand
book: pleasurable beauty evidently relates to things 'artistic' (but not necessarily art) in the sense of
adornment, embellishment, refinement, amusement, that are so to speak 'noncommittal', i.e. they
exist, are made and enjoyed for the sake of enjoyment, pleasure and the enhancement of life
generally. They are not to be despised on that account, though they are all too often misused.
Admirable beauty, on the other hand, is a metaphysical concept and relates as much to truth as to
value. A thing of beauty in the former sense need not be a thing of value; but in the latter sense it is,
and you'll discover, if you haven't already done so, that Santayana's theory of beauty is in fact a
philosophy of value. Now I know that Santayana tends to look at metaphysics with a squint in his eye,
but truth and value are themselves metaphysical concepts, in other words, they are creations of the
human mind that were not discovered in the environment or (as a present day fad would seek to
convince us) have some bearing on evolutionary biological trends, on feeding, reproduction and so
on. Beauty, truth and value belong among the staples of human proclivities that are frequently
pursued for their own sake, not for gain, prestige, wealth etc. This is what he means by admirable
beauty. It is strong and implies deeper and more meaningful connections — sub specie aeternitatis.
The rest, I think, you should discover yourself!