Knock knock, who's there? It's the history of London's most interesting doors

There is a door to a residential home in East London that is designed like a sofa. It's white leather and padded. You have to wonder how useful it is from a sound perspective, says Cath Harries. That padding is so thick that visitors must surely stand outside knocking pointlessly for hours.

Harries, 54, is the photographer behind a new book, Doors of London. She began the project almost by accident. In 2010, while working on a book about the best pubs in the capital, she found herself frequently drawn to – as well as photographing – their front doors. After a while she was 'obsessed', travelling to areas she would never normally visit just to go door hunting. (Deptford, apparently, has excellent doors.)

Often, Harries would revisit her favourites again and again to take more photos. 'I'd think, "I know I've got [a picture] but I'm just going to have another look." Because sometimes there would be a different flower outside, or a bike on the street – and if the light's nice then I just can't help myself!'

After nine years, having accumulated around 3,000 pictures of doors, Harries approached a publisher. They accepted and paired her up with Melanie Backe-Hansen, a 48-year-old historian and author of several books about British houses, streets and squares. Harries supplied the images and Backe-Hansen wrote the accompanying door-related descriptions; it was a match made in architectural heaven.

I meet the duo outside Liverpool Street station, to go on a door tour. The exact number of doors in London is unknown, but Backe-Hansen reckons there must be millions across the city. Today, we will see around 15. First stop is a black, spooky-looking door on Folgate Street replete with a gas lantern; then a navy door called 'Eleven and a Half', due to the street's renumbering at some stage; then a peeling pink door dating from 1723, which has been used as a filming location for the Idris Elba TV drama Luther; and more, and more, and more.

At a certain point, I ask Harries and Backe-Hansen whether they have a favourite. Neither can pick just one. Harries loves the aforementioned sofa door but also adores an art deco door in Forest Gate that has a giant glass panel depicting the sun. Backe-Hansen is more trad: she likes the doors along Queen Anne's Gate in Westminster, surrounded by ornate carvings and dating back to the early 1700s; ditto the doors on Albury Street, Deptford, with origins in the same era, as well as door brackets featuring floral and cherub carvings.

Now for some superlatives: the tallest door in London – and the United Kingdom – is more than two storeys high, green and located near Tottenham Court Road. It was constructed in 1904 as the entrance to a studio and warehouse for theatre props. That's the reason for its height; to allow towering stage sets to pass through.

The oldest (in London and, again, the UK) is a wooden door in the passage leading to the Chapter House of Westminster Abbey. It was constructed in the 1050s, using five large vertical planks of oak bolted against horizontal iron rails. A study showed the wood was likely from Essex, and from a tree that would have been felled some time after 1032. Looks-wise, the door is simple, without any carvings or decoration, and is, as Backe-Hansen comments in the book, 'horribly draughty'.

The most expensive? Neither Harries nor Backe-Hansen is sure. But the Giocanda Shine Door, by Pinum, a window and door company in Romania, can be bought for around £27,000. It's made from brown 'ecological leather', depicts the Mona Lisa, and is studded with 31,707 Swarovski crystals. And that's a bargain compared to the world's most expensive door handle: designed by Italian specialists Fratelli Razeto & Casareto, the gold-plated fitting is coated in diamond dust, backlit by programmable LEDs and comes in at £85,000.

It's hard to know exactly when doors were 'invented' as a concept. 'I can only guess that it's one of those things that slowly evolved over time,' says Backe-Hansen. Their purpose was probably practical, 'to keep cold and wet out', and also security. While sources vary, many claim that the earliest mechanical door lock was devised some 6,000 years ago.

Other features came later to the UK. Knockers, for instance, were invented by the ancient Greeks (it is said that people chained their slaves to the rings) but they only began appearing on residential British doors in the 16th century. Originally they would have been plain black wrought-iron rings but, by the 18th century, improvements in metalworking meant they became more jazzy and symbolic. People had wreath-shaped knockers to commemorate the Duke of Wellington's victories, and lion's-head knockers as symbols of power.

We didn't have letterboxes in Britain until the mid 19th century – before then the postal system was such that the recipient of a letter had to pay to accept it and all letters were therefore delivered by hand. When that changed, letterboxes were introduced. The early versions would have been much smaller than today's because, says Backe-Hansen, old-fashioned letters didn't have envelopes – they were simply small pieces of paper folded and secured with a wax seal.

As for construction materials, that solid oak Westminster Abbey door is now an anomaly. Today, the domestic variety in London are mostly made of veneered plywood and timber (if they are from the postwar period) or a combination of hardwood, aluminium and un-plasticised polyvinyl chloride (uPVC). Which is boring, but probably less draughty.

The one door Harries didn't photograph but wishes she had is the one in Downing Street. 'I didn't get permission! It's a real shame because I wanted Larry the cat outside it. That would have been the money shot.' The PM's front door, Backe-Hansen tells me, was once solid oak but was replaced with bomb-proof steel after a nearby IRA attack in 1991. Recently, Sir Keir Starmer bought a kitten and lamented that installing a cat flap in a bomb-proof door was 'proving a little bit difficult'.

Some types of doors weren't worth photographing at all. Towards the end of the tour, Harries, Backe-Hansen and I walk past a gigantic new-build office block with an electronic revolving door. What is the party line on those, I ask.

There's a considered pause. 'I don't like them,' says Harries.

'They're sort of a corporate development,' adds Backe-Hansen. Harries nods. 'They make sense. They're practical.'

They're just not very nice to look at.