[dorchadas]

Personally, if you're interested in the early 'philosophical' side (which I don't think can - or should - be disentangled from the 'religious' side; the separation of religion and philosophy is inherently a post-Enlightenment, Western phenomenon, and doesn't really apply outside that cultural situation), van Norden's chapters on the DDJ and the ZZ in his book I quoted from above are a good start. But there was a 'religious' tint to a lot of this, even very early on.

If you're more open to how the three strands ('religious', 'philosophical' and 'literary') of Daoism have merged and mingled throughout history, I really like Ronnie Littlejohn's Daoism: An Introduction published by I.B. Taurus. It's essentially an introductory textbook, but does a good job at showing how these things have always been interacting (indeed, there's quite possibly Nedian references in the DDJ/ZZ/LZ!) and that trying to separate them really isn't possible (indeed, as said, it's an inherently modern, Western distinction between 'philosophy' and 'religion'), while looking at how they've changed over time.

[dorchadas]

Oh I forgot to mention Coutinho's An Introduction to Daoist Philosophies, which looks at the three early Daoist texts -- the Laozi, the Zhuangzi, and the Liezi. I haven't read it yet, but have heard decent things. He specifically works from a comparative perspective as well, which makes it more interesting.