Gonzalo, you seem to be right. There is an obituary of Alexander Thom  
in the Journal of The Statistical and Social Inquiry Society of Ireland
Vol. VII, Part LVI, 1879/1880:

http://www.tara.tcd.ie/bitstream/2262/8402/1/jssisiVolVIIIPartLVI_0508.pdf

It is said that in the *History of Aberdeen*, Alexander's father  
Walter described himself as the author of the *Sketches*.

It would be interesting to understand by what kind of process the  
Kress catalogue attributed the work to someone else. Perhaps, to make  
double sure you may try to contact the publisher of the Sketches,  
Longmans: they may well have kept the records.

Daniele Besomi