The very best information on crucians is in A History of Scandinavian Fishes: B.Fries, C.U. Ekstrõm and C.Sundwall; Second Ed. revised by Prof. F.A. Smith: Stockholm: P.A. Norstedt & Sonër 1893-95.
I believe that hybridization with goldfish in the UK took place far earlier than most people realize. In Scandinavia, though, this seems unlikely at the end of the 20th century, when the book was written, and I believe that the scientific description of the crucian there can be relied upon much more than other writings. The illustrations are magnificent.
Also very useful is The Fresh-Water Fishes of Europe: H.G. Seeley, F.R.S. etc. Cassell and Co. 1886.
A great book, well worth reading for its style and content, is A History of the Fishes of the British Islands: Jonathan Couch 1860 - 65. It's quite good on crucians (don't miss the anecdote on the "Hamburg carp" in the section on the common carp) but it's worth reading from cover to cover anyway. I first read it in the British Library. It's chunky and packed with facts and stories about our fish.
All three of these important books can be accessed free of charge on one of the wonders of the Internet: the Biodiversity Heritage Library
Crock of Gold, Seeking the Crucian Carp by Peter Rolfe
This is the only book devoted entirely to the species and was published in July 2010. It is available from MPress (Media) Ltd on 0845 408 2606 or at Calm Productions.
This sometimes controversial book is the first devoted entirely to the crucian carp. It is about fishing for crucians big and small, and there are contributions from crucian enthusiasts, some famous, some not. However, it goes much further than that.
It looks at the range of waters where the fish can be found and goes into considerable detail about the best way to manage fisheries in their interest.
Peter Rolfe examines the thorny problems of identification and hybridization, the crucian’s history in this country and the way in which naturalists and anglers of the past have written about a fish that has been under-valued by most anglers. Most importantly, it makes a plea for the conservation of a species under threat and suggests practical ways in which this can be achieved.
The Net on the Garage Wall by Peter Rolfe.
The Net on the Garage Wall is a delightful exploration of one Wiltshire man's lifelong obsession with angling: tench, carp, roach and barbel share space with landscapes and journeys, insects, birds, remote ponds and the renovation of Victorian lakes. Beautifully illustrated by Stella Cork.
It was published by Medlar Press in 1997.