As salaamu alaikum wa rahmatullahi wa barakatuh...
(May God's peace, mercy and blessings be with you)
One of my favourite books on Islam (and one of the most profound introductory works) is The Vision of Islam, written by Sachiko Murata and William Chittick. Although there are many good introductions to Islam, this one is special for a very important reason. That is, most such books describe Islam, its main ideas, concepts and practices clearly and well. However, these descriptions are often somewhat external; they focus on describing what Muslims do and believe without attempting to explain why. Vision, by contrast, refers to the 5 Pillars, but as part of an attempt to explain the fundamental religious concepts of Islam. It thus comes highly recommended.
I was reading it on the train this evening. I came across a passage that struck me. I know I must have read it before, but for some reason it seemed especially poignant today. I thought I'd share it with you...
'Most Muslim thinkers hold that knowledge of tawhid pertains to what it means to be human. It lies in the original human nature (fitra) since human beings were created knowing that 'there is no god but God'. The prophets were sent to remind them of what they already know. Hence, to associate others with God is to go against the most fundamental instincts of the human species. It is, so to speak, to betray human nature and even to leave human existence. This explains why it is such a grave sin: It is the overturning of what makes us human. In this view of things, claiing ignorance of tawhid is tantamount to claiming not to be human. In the next world, paradise is the human realm, while hell is the realm of those creatures who began as human beings but did not live up to their humanity'
(The Vision of Islam, page 50)
Ma sha Allah, this is probably the best description of the meaning of tawhid I've read in English. It's simple without being simplistic and is of profound meaning. Inspiring stuff.