This page does not list every source available, only the most common and/or reliable. I will be happy to provide fuller details for anyone thinking of buying some of the more expensive books.
The information on the period is heavily dependent on one or two original sources. If Shenk made an error, then many others will repeat that. All of them, and this site is no different, give the official uniforms. What was actually worn might be very different.
The Russian Civil War (1) White Armies, by M. Khvostov, plates by A. Karachtchouk, "Men at Arms" #305 (Osprey)
An unreliable book, with several major errors and patchy coverage, concentrating on glamorous and unusual units. Not helped by a very Soviet definition of "White" i.e. pretty much anyone other than Bolsheviks.
The Russian Army 1914-1918, by N. Cornish, plates by A. Karachtchouk, "Men at Arms" #364 (Osprey)
A good introduction to the period.
Handbook of the Russian Army 1914, by the General Staff of the War Office #364 (Imperial War Museum/Battery Press)
A reprint of the British Army's handbook on its Russian ally. Useful because it covers the subject in much more detail than most other sources.
Flags and Standards of the Russian Imperial Army, by T. Shevyakov, with art by O. Parkhaev (Gauntlet, original Russian version by AST)
Little direct relevance to the Civil War period, but a guide to understanding Imperial practice. The bulk of it is concerned with the ceremonial banners.
Civil War in Russian 1917-1922, White Armies, by A. I. Deryabin, with plates by R. Palasios-Fernades (Gauntlet, original Russian version by AST)
Concentrates almost exclusively on the glamorous coloured units, but has good detail.
Uniforms of Imperial and Soviet Russia in Color, 1907-1946, by H. Knotel (Schiffer)
A large number of plates of Imperial uniforms. Focussed mostly on dress uniforms and guards, and there is no text describing the plates, so many details are impossible to distinguish.
Army Uniforms of World War I, by A. Mollo and P. Turner (Blandford Colour Series)
Only a dozen pages and 19 plates on Imperial Russia but it does show Greek, Czech and Polish uniforms as well. A good book, but only really an introduction.
Uniforms of the Imperial Russian Army, by B. Mollo and J. Mollo (Blandford Colour Series)
Covers too big a time span (1699 to 1917) and has excessive emphasis on the parade uniforms and elite units in the 28 drawings for WWI. Small picture size doesn't help either.
Uniforms & Equipment of the Czarist Russian Armed forces in World War One: A study in Period Photographs, by S. Coil (Schiffer)
An expensive book with some good photographs of WWI-era Russian equipment, sadly badly let down by the erratic layout and obscure commentary. For afficionados of militaria only.
Farewell to the Don, by Brigadier H. N. H. Williamson (Collins)
Useful for eyewitness accounts of some period detail.
The RCW 1917-1922: White Armies, by A. Deryabin, illustrated by R. Palasios-Fernandez (AST)
The Russian equivalent of the Osprey. Again it focuses excessively on a few well-attested elite units, but its coverage is better and it is far more reliable. An English translation is available.
White Russia Photoalbum, editors V. Zh. Tsvetkov and B. S. Pushkarev (Posev)
Hundreds of period photographs.
White Guard Almanac #8 (Posev) a collection of articles which includes:
The Anti-Bolshevik Movement in the Astrakhan Cossack Army, by O. O. Antropov
Uniforms, medals, flags and signs of the White Cossacks, by A. A. Karevskiy and A. V. Lebedev
Details on little known units.
Encylopedia of the Civil War: the White Movement, by S. V. Volkov (Neva). Much of it can be found at http://swolkov.ru/bdorg/list-num01.htm
The main source for unit histories, concentrating in the south.
White Guard, by V. V. Klaving (Ol'ga)
Not as detailed as Volkov for individual units in the south, but full of orders of battle, and better coverage for the minor Cossack states out east.
Marching and Horses, by Serge Mamontov. Available in French as
Carnets de route d'un artilleur à cheval 1917-1920 (L'Harmattan)
Useful for eyewitness accounts of some period detail.
Uniforms of the Russian Army, by V. K. Shenk available at http://www.xenophon-mil.org/rusarmy/shenk/shenklist.htm
A primary source for many uniform sources above, so if several sources are in agreement it might be that they are all based on Shenk.
Flags of Russia, available at http://www.vexillographia.ru/russia/index.htm
Some good material, but mostly taken from resources given below.
Banners, standards and flags of the Russian Imperial Army and Fleet in the Great War 1914-1917, available at http://mosgrenadier.narond.ru/dip/dip0.htm
The standard text on the subject. Most sites rely on this material, so an error made here will propagate widely.
Wikipedia in Russian http://ru.wikipedia.org the phrase "Гражданская война в России" will get you started on the Russian civil war.
The usual Wikipedia mix of good and bad, but some obscure areas are covered.
S. V. Volkov's site at: http://swolkov.ru/bdorg/list-num01.htm
The same material as his Encyclopedia, but the ability to autotranslate can speed things up.
Bergenschild site at http://www.bergenschild.ru/Reconstruction/depot/civil_war/Index.htm
The front of the site is devoted to a couple of re-enactment groups. Behind is a wealth of material relating to the White armies in the east, including material on KOMUCH and lesser known units.
Kolchakiya site at http://www.kolchakiya.ru/uniformology_main.htm
Immense coverage of the uniforms of the White armies in Siberia.
Diorama.ru and in particular this page https://www.diorama.ru/workshop/reviews/147/
The comments and links are as useful as the material directly cited.
White Guard is one of the better re-enactment sites, particularly for the RCW http://www.white-guard.ru
An on-line translator and you're away, although you need to know what things are as there are no descriptions.
A-pesni is an Anarchist site http://a-pesni.org
Mostly devoted to songs of the period, but tucked away in a couple of places are a whole bunch of articles, often on quite obscure subjects.