Yes, the "Imperial Army Series" of manuals must have been a nice little earner for somebody. I haven't actually compared them with the 'official' manuals, but they are extremely similar in content (probably just enough differences to avoid official notice).
There are some very early official manuals about, I've got Signalling Regulations 1904 and its Indian Army counterpart, plus various copies of the Signalling Training 1907, 1909, 1911 and 1914. There's also a thinner book "Signalling Training Part II" which appears to be a cut-down version for the Territorial and volunteer forces in the UK.
They'll get scanned at some point, but are rather fiddly to do because of the small size (and hence the need for high resolution scanning) requiring them to be done by hand.
The late/post WW1 manual set (1917 - 1920) were all "Signal Training Part (number)" and I'm trying to complete my set. Some are quite common (Part I), others very much the reverse (Part IV - Instruments) and I've never seen Part III or Part V for sale anywhere. (I was outbid on the Instruments one, some years ago.)
The name changed again around 1923 to "Signal Training Volume (number)", sometimes divided into 'parts', and with Volume III as a series of pamphlets because a single volume would be permanently out of date as more equipment came into use. The Volume III pamphlets appear to have ceased being numbered after the WS18 pair, and then they switched to "Working Instructions" and reference cards shortly afterwards.
Chris.