With a safari subscription you pay about $50 a month.
Great you get 5 tokens to use a month which 'purchases' you downloads.
However if you don't use those tokens in 90days you lose them.

A book 'costs' between 10 and 15 tokens on average for a 'useful' manual or freference one.
The books are available on Amazon or other web stores for less than $25.
Save your subscription and buy 2 books a month.