There is (usually) a rhyme and reason to the logic of English - if only you can figure it out.
So consider the difference between these sentences:
(a) This book is computer related.
(b) This is a computer-related book.
(c) This book is computer-related.
(d) This book is related to computers.
(e) This book relates to computers.
In (a) we have the origin of the adjectival expression where computer is a noun and related is arguably (part of) a verb (in past participle form, in a periphrastic verb construct we prefer in English to the simple present - compare (d) and (e)). On the other hand the PP "related" is arguably acting as an adjective, and computer as a modifier to that adjective. Thus (a) is a transition form between a verbal usage (like (d) and (e)) and an adjectival usage (as explicit in (b) and (c)). The form (c) is conventionally not used as it is not necessary to force it into an adjectival mode as the verbal mode of (a) suffices, and indeed is what originally licensed the compound adjective).
We should also consider the pragmatics, as in any given context usually one specific one of the above variants would be preferred and would seem more appropriate. I would however, very seldom use a form like (c), but consider - "this book is theoretical but computer-related", or "this book is theological but this one is computer-related" where the conjunction with an adjective forces the adjectival reading (given there is no existing single word adjective), and hence the compounding into an adjective. Also (d) is a little unlikely as "this book is about computers" is more likely, and "related" would be awkward as it is topics that are related not books, and the metonymy is lost here out of context. It is possible the book is not all about computers, but about a topic that relates to computers, so (d) or (e) could occur - e.g. if you are looking at psychology books, trying to find one that include an element of computational modelling - I think I would prefer (e) in this situation, or better still "this book makes the connection with computers".
When you get home and voluntarily show off what you bought is the time you might use (b). But if you were asked to explain the relevance of the books you bought you might use (a), as part of a list.
Short (but controversial) answer: (a) doesn't have an adjective.