If you're still interested in learning HTML 3.2 then it is of educational value. But if you already know 4 and are proceeding to learn 5 (or already have), then it isn't of any educational value to you.
There is no real reason to learn a standard that is no longer in active use. HTML4 is certainly still valid, but you'd be better reading something dedicated to the subject (or even HTML5) than a book that focuses on something that has long been deprecated and does not teach modern web development standards and principles. The trouble with learning 3.2 is that you'd have to unlearn half of it before getting your head around HTML4.x and HTML5.