The Panzer Tracts series was started by Thomas Jentz and Hilary Doyle in an attempt to correct many of the mistakes and misconceptions about Panzers that exist due to poor research. By using only primary sources, it is ensures that all of the information is correct - this makes these books the ultimate reference in many aspects.
In the beginning, the books were somewhat shallow in contents, and had a somewhat crude appearence. The more recent editions have grown rapidly, though, and the paper quality has increased rapidly. The details contained in the books makes them a must for all serious students of the Panzertruppe.
The planned series expands over 20 titles, some of which are divided into several editions, ranging from the Pz.Kpfw. I through to the Tiger II, as well as editions on topics such as Sturmartillerie, Flakpanzere, Beobachtungspanzere and so forth. Just about every German AFV would appear to be covered once the series is complete.
Besides these 'standard' editions, there are also titles on special subjects, such as the Maus, the Karl Gerät and Rommels Funnies.
All of the books contain very interresting information, and are highly recommended. The only downsides are that some of the early volumes does not contain much text (however we can hope that these titles will be expanded in the future), and the price of about $20 per title, which will make the price of the complete series quite expensive. Considering the information you get, and the work done by Jentz and Doyle to gather this information, the price still seems very fair.