Rig Veda's main theme is basic knowledge covering a wide range of topics as concepts, qualities and characteristics of God, soul and Prakriti (the primary source of nature). It also covers the properties of the visible and invisible world, and some basic aspects of physical sciences as chemistry, physics, mathematics, astronomy, cosmology, etc. It also contains human ideals and principles to be adopted in all walks of life. In words of Sayana, Rig veda contains nature, properties and activitiesof all types of materials, i.e., physical, metaphysics and spiritual.
Rig Veda is the largest veda in terms of language content. It has 10 Mandals; 1028 Suktas; and 10581 Rik Mantras. In the last century, many books have been published on Rigved including the Mool (main sanskrit text), Bhashya (translations), Teeka (commentary) and Vyakhya (descriptions) on the whole or part of the Veda.
Rigveda with commentaries of Skandasvami, Udgitha, Venkata Madhav and Mudgala; 8 volumes by Vishwa Bandhu. Published by Vishveshvaranand-Indological-Research, Hoshirapur in 1966. Available at http://www.saujanyabooks.com/details.aspx?id=11085&Rgveda-with-Commentaries-of-Skandasvamin-Udgitha-Venkata-Madhava-and-Mudgala_8-Volumes/Vishva-Bandhu
Asvalayana-Samhita of The Rgveda (2 Vols.) (With Padapatha, Detailed Introduction and two Indices) by B.B. Chaubey available with http://www.indicabooks.com/Books.asp?category_id=9