AJ, I have studied hinduism, and not through Hindu books, but from Muslim books, compiled by Muslims Soofia and scholars. Please do not get confused with the word 'god' or 'deota'.
god IS NOT EQUAL TO God
Their 'gods' are more like our 'angels', and 'prophets'. I wish I could post some kalam from 'Kabir' who talked about monotheism or what we call 'Touheed'.
As far as my understanding and limited knowledge is concerned, the Hinduism is also a monotheistic religion, the confusion is because of the names and the terminologies.
I'll also add that their (Hindu's) deities are even considered 'aspect' of one supreme being (Oum, or Allah). As you know, for Allah, there are ninety nine (99) names, and each name is one individual 'aspect' or 'quality' of one supreme being (Allah). Rahman is Allah, so is Raheem, so is Jabbar, so is Qahhar, so is Ali, so is Wakeel and so on and so forth. Now read the following passage from Rigveda 1.164.46:
Indraṃ mitraṃ varuṇamaghnimāhuratho divyaḥ sa suparṇo gharutmān,
ekaṃ sad viprā bahudhā vadantyaghniṃ yamaṃ mātariśvānamāhuḥ
"Theycall him Indra, Mitra, Varuṇa, Agni, and he is heavenly nobly-winged Garutmān.
To what is One, sages give many a title they call it Agni, Yama, Mātariśvan."
Now what do you say? Isn't it the same concept we have for Allah's ninety nine names?
Om (Oum) in their religion is Allah in ours. There concept of 'trimurti', which is represented by Om, in puranic hindusim is something like the Christian trinity. Trimurti is a cosmic functions of creation, maintenance, and destruction are personified by the forms of Brahmā the creator, Vishnu the maintainer or preserver, and Śhiva the destroyer or transformer. Again, you'll find similar 'aspect' for Allah in his ninety nine names.
Wallah aalam bissawab (Allah knows better).