With the characteristics that bitcoin is showing you don't need a book to tell you that bitcoin is an asset and with were you would have heard about bitcoin you should be able to tell that bitcoin is more of an asset than a currency and only on few circumstances that it is seen as a currency and only few countries can even spend it as an alternative currency.
And the value can increase and decrease anytime makes it an asset to and can be sold anytime you want, those are all quality of an asset and the most important is that it's a long time investment and bitcoin is one of a kind investment and it's community keeps growing bigger and bigger and it's one of the best investment anyone can investment and even the government are trying to also invest because look at how El Salvador have been succeeding in investing in bitcoin.