For a quick overview of Queen's troops, volume 3 of Osprey's The British Army on Campaign MAA series is probably the easiest way to get started. For Indian units, I believe the two MAA's on Bengal infantry and cavalry regiments are still in print. Being Ospreys, these booklets are far from exhaustive, but they at least cover the basics in reasonable detail.
I agree that Boris Mollo's The Indian Army is a great ressource, although I'm not sure it's been reprinted lately. I was lucky enough to find a copy at Foyle's aeons ago when the book was already fetching steep prices on the second-hand market. That said, don't hesitate to grab a copy if you can find one : it's worth every penny you'll pay for it.
Ian Knight's Go To Your God Like A Soldier is not a uniform guide, but it is chock full of period photographs and engravings, and so is Soldiers Of The Raj, published some twelve ago by the National Army Museum. Dunno if the latter is still available, but both are very nice pictorial ressources.