RHS encyclopedia will be a good reference. However as a keen gardener, who has moved many times, may I make some other suggestions. Firstly you need to live with the garden you have now for the first year. Having a simple notebook , where you can note what grows where, and the time of year things flower. Also , draw up a plan of the garden and the various areas, where you can then note where it is in sun or shade at the different times of day and in various seasons. Doing this will give you an overall knowledge of the garden and how the shady areas and sunny areas are . Taking simple photos will also give you the overall view to look at later. Then very importantly, test the soil in the garden in various parts of the garden as it can be different in the areas in the garden. When you know whether your soil is acid or alkaline and the type of soil - sandy or loam . These are your building blocks on the way to a great garden. When you know this you will then have better ideas of suitab le plants and where they are going to do well. Much more likely to succeed than buying a plant and not giving it the type of soil or light that it will prefer. Walk up and down the road and look at other gardens. You will get a good idea of what does well in your area. Best of all look around for the local gardening club and join it if you can. You will meet other keen gardeners and have the chance to buy and swap plants and get a lot of help and knowledge of the local area. So let the garden show you what it has , keep the photos so that in years to come you have a view of how it was when y ou arrived . Right now when they have just moved with lots to do so I would suggest that they get 2 or 3 pots, plant them up with miniature daffodils, or iris reticulata to give them something to look at close to the windows in the new year and plant some violas or pansies in one pot to give you something to enjoy right now. I have always enjoyed having a simple largish notebook where I didnt write every day, but did make a note of the date and what I have done that day, and what the temperature is like. Then I can look back over the year and see what worked well and what didnt. So I think a decent notepad and either a general rhs reference book, or if you know she3 is very keen on a specific plant find something on that. I am a galanthophile - snowdrops - and had over 100 different kinds in my last garden.
One last thing I think is worth doing is every Christmas day and New Years day I take a walk round the garden, Walk round to the left, then turn round and walk to the right. You will be surprised at how things can look different from each direction. I make a note of any flowering plants in my notebook. Have kept notes over 50 years and did move a lot but spent nearly 20 years in a later house and you can see how things flower earlier or later. It is my tradition and I go out whenever it suits me. Everyone knows I do this and so if things are getting a bit fraught with visitors or whatever, I can just announce I am going out to look round , which gives me a breather and no one gets upset! It is very calming and cheering to go out for a little while. I wish them the joy of their new home and garden .You never stop learning and enjoying your garden.