This section is not specific to Essex maps, but rather applies to all UK antique maps.
It is sometimes said that collecting maps is a hobby open to many people, as you can spend thousands of pounds, or just a few pounds. There is certainly a massive variety of prices out there. I personally have just always found maps fascinating, and am very happy to combine appreciating them for their artistic and technical abilities, what they tell you about the past country and the past's attitude about what to show on maps, plus just being a collector happy to horde some special type of property.
There are a vast number of possible maps to collect, so you need to start by deciding just what is your field? You can look at it by geographic reach, eg estate/parish/town vs county vs country vs continent, and/or by century, eg 16th/17th/18th/19th/20th/even 21st.
I think you need a trigger - a specific cause and a reason - to decide on what you are collecting. For me it was looking at the Chapman & Andre wall map in the Saffron Walden Museum, and seeing how the main road from Audley End up to Littlebury is "wrong". There and then I said to my wife, "I could collect all the maps of Essex before the Ordnance Survey".
I collect Essex from every period; the first OS is 1805 by the way. Can't say I've collected a 16th century one of Essex though, as that is Saxton 1576, and that's £3,500++. Alternatively you may seek to collect all the maps by one cartographer, eg Kitchin's atlases and one off maps of the latter 18th century.
17th century maps from the Netherlands are several hundreds of pounds per county; 18th century maps are say £50 to a few hundreds per county; 19th century county maps are £10 to £100; 20th century £2 to (say) £30. On the whole, the older the more expensive, plus the larger the more expensive.
Not surprisingly atlases cost more, and of course that has been the source of many of the individual county maps available today - atlases that have been broken up into individual sheets. This is not true of all of them though; from the time of Speed the map sellers have sold individual sheets, deliberately designed to be decorative as well as informative.
So having selected what you want to collect, how do you find out what there is available, and where do you get it?
I think you should try to work out just what is available of your selected topic; create a listing of what there is (see the Mapmakers page), and then you can a) mark down items as you get them and b) keep a note of where something may be available (& at what price). How do you work out a listing?
The starting point is Books. Whatever is your particular topic, there is very probably a book on it, or at least books that refer to it in detail or in passing. For instance, there are books on the British County Atlases, from earliest times to c.1850 or 1900 (see the Books page). If your interest is in one county, there are for some counties detailed books listing all the known maps of that county, again typically from earliest times to c.1900 (see the Cartobibliogs page). For the Ordnance Survey there are several books, and indeed a society - The Charles Close Society - for information on what there is.
Then there are the major County Libraries and copyright libraries, and the County Record Offices. Essex has an excellent county record office down in Chelmsford, with a terrific collection of maps of all types, and all the county atlas maps are now catalogued. They have half a dozen Speeds, Bleaus and so on, and all are visible on their electronic index.
Each county has its own County Record Office - most of them less high tech than Essex though, with Suffolk and Herts looking as though they have not changed for 50 years. There are also the top university map depositories, such as Cambridge University Library Map Dept, and the British Library, which has the best collection in the country (see the Libs & Socs page).
The main places to visit are the dealers and fairs. Most antique dealers have the occasional old map, and there are a few specialist map dealers around the country, about 40 to 50 of them. The antique map dealers are the real specialists who are generally delighted to talk about the maps. There is only one specialist antique map dealer in Essex and none in Cambridgeshire; London is of course the place with the most specialist dealers, spreading from the West End across to Knightsbridge. However, you don't need to start by visiting all these places, as they all have internet websites, and that's the place to start - their web sites (see the Dealers page).
They generally have sites where you can search by a key word, and/or they have catalogues sequenced by cartographer and/or geography. I have a listing of 40+ map dealer web sites that I trawl through about once a month nowadays, looking for those missing ones. Sadly the dealers do not always have all their stock on their web sites, so it can pay to visit them when you have run out of items to buy from their web sites.
Talking about the web, a major place to look is eBay. It's not a place to find genuinely old items, but for 20th century items it is probably the best place to look. I mainly buy OS, Bartholomew, Geographia and Bacon maps from eBay, dating about 1890 to now, and costing £2 to £10 or so. I have bought older maps, but only if really cheap - ie notably cheaper than sold by dealers - so that if it is not genuine I'm not going to be losing too much. I would not pay £100s for a map via eBay; there has to be a risk of forgery.
A final place I'd look is fairs - car boot fairs, antiques fairs and antique book fairs, and even antique map fairs.
Car boot fairs could be a place for old car atlases, OS maps and similar. But anything else seems unlikely. Antique fairs generally have some maps, and these can be 20th century OS back to 17th century Bleaus. Antique book fairs can usually be relied on to have a few stalls that have many antique maps, with stall attendants who may know about their stock. At the Antique Map Fairs there are the real specialists, basically the dealers I was talking about earlier, but with many from abroad there as well.
The antique book fairs happen monthly in London, and once or twice a year in Norwich, Chelmsford, Bury and Cambridge. The Cambridge one is a two day affair, with a lot of stalls and a fair number of maps and similar available; it takes place each year in February. The Antique Map fairs are only once or twice a year down in London.
One of the major advantages of buying from the antique map dealers is that they will have dated the map, and accurately. Trying to date a map yourself can be quite a problem. The OS have always dated their maps, often showing an initial release date of the edition and the date this particular copy has been revised up to. Some other maps have a date on, but often need to be treated with caution.
For instance, the Speed maps were first published in 1611, and kept saying 1611 in subsequent printings, all the way up to 1662, when it then was changed and kept that date for the next 50 years. Most antique maps just don't have a date on them. Happily, most maps were only published once, and those that had multiple publishings usually had some small change which you can then use to date them, based on the info in the reference books.
In the 19th century you might think that you could use the spread of the railway lines to date things, but that is very risky. The snag was that if a cartographer wanted his map not to get out of date too quickly he would include projected rail lines - and some ended up looking very smooth on the maps, far smoother lines than were actually built. For instance, the development of railways in Essex starts in 1839, by 1842 the line gets up to Stortford, 1845 from Stortford past Saffron Walden to Cambridge, and 1868 when we got our branch line from Audley End to Saffron Walden (& on to Bartlow in 1869 - see the Railways page).
As an example, one of the most common maps you will see are the "Moules", originally published in 1838, and continuing to have that date quoted despite having a lot of railways shown on them. From the amount of railways on them, they are generally 1850s/60s.