Some of my favourite books are the magnificent series on Romanesque architecture produced by Zodiaque, at La-Pierre-Qui-Vire in France.
Each book in the 'Nuit des Temps' series focuses on a single region - some regions get more than one - and though Zodiaque is a French publisher, it doesn't neglect other countries. Leon, Castille, and Catalunya in Spain, even Scandinavia and the UK, have books devoted to them. There's even 'Terre Sainte Romane' -Romanesque work of the Crusaders in the Holy Land (let's not ask too closely what that means in terms of modern state borders).
Romanesque architecture isn't a single style; each region has its own. There's the Herefordshire school with its ornate, almost barbaric carvings; the austere, geometrical Norman style of Caen or Norwich; the elegance of Poitevin work, or the classical echoes and monumental aspirations of Burgundy.
That's one reason I love these books. They really get to the heart of what makes each regional style different.
Another reason is the superb, mainly black and white illustrations. It's very difficult to take a single photograph that will please the art historian and still convey the architecture of a place, but somehow Zodiaque manages to pick the right ones.
These books aren't intended to be gazetteers - there's a separate range of 'Itineraires romanes' also published by Zodiaque for that. But they are complete enough that you could arrange an enjoyable tour around the contents of any one book.
What's the downside? They're not cheap - thirty euros or more, most of the time. And they're not easy to find. Many of them are now out of print, and there are other collectors besides me - I remember visiting one French fan when I was walking to Santiago who had an entire book case full of them, and since it was full he'd started stacking the new ones on the floor!
Sometimes I manage to find them in bookshops on my travels, and even if I'm travelling light, there'll be room for them in my rucksack if I do! Occasionally I find one on ebay. And just once I was lucky enough to find one in a jumble sale.
Intrigued? There's a whole site (in French) dedicated to the Zodiaque books.
Just one warning. Don't become an addict, like me. It can get expensive.