You should read and understand the Place Value and Fractions books first.
Decimal Fractions are an extension of those two topics and if you do not really understand them you will not fully understand Decimal Fractions.
If you do know and understand Place Value and Fractions, Decimal Fractions are EASY.
You should also learn the Integers book before you learn this one, because in this book I assume you understand integers, positive and negative numbers, and numbers in general.
If you studied those three books, Decimals would take you 5 - 7 hours, and by the end of it you will know everything you need to know about Decimal Fractions.
Here is a short excerpt from the book:
Decimal Fractions are Fractions that are written with the Decimal Place Value system.
This is why Decimal Fractions are often called “Decimals”
In fact, all the numbers we use are “Decimals”, which means they are numbers in the base 10 system.
Because a Decimal Fraction is written with the Decimal Place Value system, it is a fraction with a denominator that is a multiple of 10 (10, 100, 1000, etc.).
Similarly to fractions, you can have mixed numbers (a whole number and a fraction together) or negative Decimals.