Umm, well I am an Iyengar teacher actually so erm, all my recommendations will come from there I am afraid. Though I think Iyengar has an unfair reputation for being a specific "style", all teachers are different and there is a massive amount of variation even within Iyengar yoga.
There are a couple of amazing Indian teachers due to be visiting Kent in the next 6 weeks. Firooza Ali will be there in May, as well as being exceptionally good at teaching postures she has an MA in Philosophy so philosophy is interwoven with her teaching. www.kentyoga.org.uk/item/1167489
The alignment of Iyengar yoga leads in to the philosophy but it's implicit rather than explicit. Once you've experienced it, you have to learn how to "read" it if you see what I mean. But it is very difficult to study alone.
The Bhagavad Gita is a good place to start, I recommend Eknath Easwaran's translation and commentary. The Yoga Sutras of Patanjali is a key text - Edwin Bryant and Gitte Bechsgaard have written excellent commentaries (though not cheap). Light on the Yoga Sutras by BKS Iyengar is also a good place to start with studying the Sutras.
The first yoga sutra is (translations differ!) "Now begins the practice of yoga" - so questions of what "now" means, why now and not in the past or the future, what is yoga, what does practice mean, what does beginning mean, why do this practice of yoga etc are all ideas to explore both theoretically and on the mat, and in life.
That's the simplest of them as well - keep going and you'll always be questioning why you're doing what you're doing in the way that you're doing it...