Apologies that my posts are SO long, but its all the info I wished I'd known when I inherited my aga. I too have a Mary Berry aga cookbook which I found in a charity shop, but there are lots on ebay too. TBH, I rarely use it now though.
The previous owner might leave some of the accessories for you. A toaster rack, trays that fit the runners, kettle etc. Our aga gas man gave us some.
These covers help protect the lid from splatters https://bakeoglide.kitchen/products/splash-shield
I'm not sure this is the correct size, but there is a circular, non stick Bake-0-glide which we put on the hot plate to make toast. There is also a metal device which you put the toast in, but we found the bake-o-glide stuff better at protecting the hot plate and less crumbs than the toaster rack https://bakeoglide.kitchen/products/bake-o-glide-microwave-liner
I highly recommend hob covers, also called chefs pads. They protect the tops from scratches, means you can get a hot pan out of the oven and sit it on top and also reduces heat loss. I have a festive design for Christmas, which were on sale in Jan, but day to day just have an all black set. They get drips and pan marks on them, so avoid anything cream/white because they become filthy very quickly! https://www.blakeandbull.co.uk/collections/hob-covers-chefs-pads-other-textiles/products/hob-cover-oak-and-bumble
I bought a 2nd rail and a rack which sits over the aga to dry clothes. I've never use our heat pump clothes dryer in 2yrs.
You'll also need a kettle. Get one with a thick base and a whistle. Our aga guy said he sometimes gets calls from people thinking their aga isn't working, because the kettle was taking too long to heat. He said that cheaper kettles, with a very thin base, can warp, so only a small part is in contact with the hotplate.
Our gas usage does go up when we turn it on, but it does replace so many items.
-toaster
-kettle
-slow cooked
-clothes drying
-drying herbs and dehydrating fruit/veg
-You can apparently press clothes across it, but I've never tried