The Plant Lover's Guide to Primulas, by Jodie Mitchell and Lynn Lawson. Timber Press, 2016
This book is pure eye candy for gardeners. It contains a huge number of photographs of both primulas close up and garden and wild settings. It lists a huge number of both species and varieties of primula, in every color you can imagine, including true blue. They include what type of growing conditions the plants want (some can be in very moist areas; others need well draining soil) and what plant companions they do well with. The only problem with a lot of these plants is that they are only available from the author's nursery in France; they do ship to the USA but of course you have to pay for a phytosan certificate.
The authors have been growing primulas from both seed and divisions for years; they are the caretakers of the famous Barnhaven strains that include doubles, hose in hose, laced, and other delights. So they know what they are doing with primulas and can give the best advice. This would make a great gift for the gardener on your list.