The gāyatrī mantra japa occupies the centerpiece of the vedic tradition. A new collection of sanskrit commentaries on this mantra is offered here. It includes the commentary by paiṅganādu gaṇapati śāstrī -- a particularly lucid and practical commentary.
For the remainder of the sandhyāvandana ritual, there are a couple of traditional commentaries
Narasimha Rao has prepared simple easy-to-use manuals with english instructions. They are short and to the point. Use them daily, weekly, or as time allows.
See his site to find additional versions of these manuals in many languages.
Simple, meaningful, regular sādhanā is the key to spiritual progress. The traditional daily rituals (āhnika) of all dvijas included dhyāna (sandhyāvandana), tarpaṇa, homa, and pūjā. Here are some traditional manuals, from the Digital Library of India.
A few authors have collected the varied opinions of śruti and smṛti on various practical day-to-day matters. One of the most celebrated of these was vaidyanātha dīkṣitīya who complied the smṛti muktā phala.