A long time ago when I read the River Cottage Mushroom handbook it explained the importance of having at least 2 mushroom guide books to confirm what you were identifying. John Wright is a wonderful writer, I really enjoyed that book, but I'm not actually that wild about mushrooms so never got another one. I do think that anybody who is at all interested in cocktails needs at least a couple of recipe books though, and sort of for similar reasons.
With a modicum of self-control cocktails are unlikely to poison you (please drink responsibly) but there's a lot to be said for being able to compare recipes for classics, and having something to cover all moods. I have a small handful of reprints of older books which have taught me a lot, and a couple of more contemporary volumes which are great too - though they mostly lean towards aperitifs, vermouth, and other things along those lines.
I have strong opinions on what makes a good home cocktail and a good home cocktail making guide. The key principle is that they have to be relatively easy to make - so no overly specialised equipment, and not to many hard to source ingredients. If I want that kind of cocktail I'll go to a bar. Richard Godwin (you can subscribe to his The Spirits newsletter (he describes it as like a book club but for drinks which it is) to get an idea of his style is excellent at providing the perfect to make at home recipe. Having followed his weekly bulletins for most the summer I ordered the book fairly soon after seeing it was being reissued.
A limited number of ingredients means you can invest in reasonably good quality spirits without spending an absolute fortune (you get out what you put in, spending an extra couple of pounds for a good mid range vodka, gin, rum, bourbon etc is worth it) which also matters. The great thing about Godwin, and this book, is that he takes what he's doing seriously, but not to seriously. There's a lot of history and information here and an excellent selection of classics - old and modern, and a whole lot of varietaions on them. Basically something for every mood or occasion.
This is another important thing about making really good cocktails at home - balance is important and so are proportions, but my idea of when a drink is sweet or sour enough might not be yours. The only way to get it right for you is to test. When you're stating out a book that actively encourages you to make adjustments as wanted is really helpful. Godwin also suggests substitutions which is possibly my favourite thing about his approach to drinks writing. This really is a book that will help you (me) make the most of what I have to hand.
It doesn't hurt that it's a genuinely enjoyable book to sit down and read too. My copy arrived looking like it had already seen some life, and in the 24 hours since it's seen some more. I've tried the Daquiri Mulata (dark rum and coffee liqueuer with lime juice and golden sugar syrup - an excellent twist on the classic) and am going to bed shortly to read up on punches - of which I'm promised plenty.
(at time of writing the best price I've seen it for online is via Blackwell's)