I have bought books steadily since 1984: most are Collins Guides (Insects, Orchids, Land snails, mushrooms and toadstools (this was the first I think!) Mosses Lichens & Ferns, Seashore, Spiders etc) and these give me a chance to identify something and also by reading and looking at the pics - a rough knowledge of something before I find it!! One of the best flower identifiers is the Wild Flowers of the British Isles by Gerrard and Streeter - its a biggie not one to carry round the fields - but I've found it best for its acurate life-size paintings and ID pointers - and a quick way to ID esp when on holiday - rather than reading thro a key (which can be done later).
For serious flower ID I have the Three Wise Men as we used to call it at school - Clapham, Tutin and Warburg Flora and their Excursion Flora for taking into the field, also Clive Stace. And both Watson and Smiths books on Mosses and Liverworts.
At our old address (which reminds me to reorder) I used to get the latest 'newspaper-type' paper from the Natural History Book Store/Shop (Totnes, Devon) advertising natural history books and this is very useful - not only having all the latest natural history books in , but a huge selection of advertising for stuff already published. It can be hit and miss by mail order and not having seen the book first - I have a huge tome from them on fungi and I cannot get into it (all scientific text and no pretty picture!!) Maybe when I retire I might read more........
Most of my library are ID-type books, but a few are about a subject rather than what it actually is: Enjoying Moths by Roy Leverton, The Private Life of Plants David Attenboro from the tv series, British Isles - Alan Titmarsh same to accompany tv series,
Badgers by Ernest Neal, British Bats - John Altringham, a range from the New Naturalist series. Also books about photographing a range of natural history subjects. And several guides to various nature reserves around the country plus stuff bought on holiday such as the Natural History of Dorset, Flora of Cumbria, Flora of Hampshire etc.
There's lots to go at 'out there' so good luck and enjoy!
Pauline