I really like Peter Wilson's book on the subject. It breaks it down into very easily understandable chunks. It is not a military history if that is what you are looking for.

Wedgewood is very good too but I find Wilson's book flows better.

There is also Schiller's history which is a little hero-worshippy. It does focus a lot more on the big events, is more narrative and less explanatory. One big bonus is that you can get it for free and much of it is on librivox.

The answer depends on the depth at which you are interested in reading about the subject.

Wilson's book is probably the best known, but I prefer Geoffrey Parker's book which I think has better flow. I also think the prose is a bit better.

I have read Gardiner's book and I think it's a fine book. However, I agree that Schiller's history is too romantic and too focused on heroic actors.

Beyond those first two overview books, I also like Geoff Mortimer's Eyewitness Account of the Thirty Years War which is a delightful first-person account of a diverse set of characters who lived during that time. It also discusses the complex task of assessing the degree of destruction that that conflict wrought.

I'll throw my weight behind the already mentioned versions. C.V. Wedgwood's The Thirty Years War is a classic of the topic, and Peter Wilson's The Thirty Years War: Europe's Tragedy is also a good read. Wedgwood's is available on Audible too, I believe, in case you prefer audiobooks, though I can't say I recommend it that way unless you have a detailed map of Europe in front of you at the time. I haven't read Parker's work on the topic, but having read a lot of his other work, he does have a writing style that I enjoy and find easy to read.