Since the issue of diacritic design came up, I thought I would recommend a book I’ve had on my bookshelf for a long time, the Phonetic Symbol Guide by Geoffrey Pullum and William Ladusaw.
The feature I like is that the book shows the individual symbols at a reasonably large size (ca 60 pts) with guidelines marking the baseline and the top of the x-height. This can be handy because phonetic transcription includes symbols that are actually small caps, particularly the uvular sounds /ɢ,ʀ,ʁ,ɴ,ʟ/ which are properly only as tall as the x-height. On the other hand the book acknowledges that phoneticians may cheat by using full cap /G,R,N,L/, especially in e-mail. Fortunately, this book doesn’t judge.
A critique is that not every possible transcription symbol is included including those from the traditions such as the Uralic Phonetic Alphabet. Fortunately, there is more information online.
The other nice feature of this handbook is that it covers both IPA (as of 1996) and common non-IPA usage. If you are reading older articles, this is VERY handy as IPA was not really standardized everywhere in phonology until relatively recently. If you read articles from the 1970s (before we all had access to cool fonts), you will see all sorts of interesting non-IPA substitutions. I myself still prefer the Spanish to type /ñ/ for the palatal nasal /ɲ/, but my kind will eventually die out.
Even though IPA is being adopted by venues such as Wikipedia, I suspect there are some hold-outs from related fields such as historical linguistics, Celtic philology, Germanic philology and so forth. It’s critical to know the IPA, but just as important to know the important variations.
On the other hand, if you are interested in learning proper IPA transcription, you should also own the Handbook of the International Phonetic Association. However, it only covers the IPA and does not show large scale representations of the individual symbols (most are shown only at 12 point). It does, however, include Unicode points (“UCS”) for the characters included in the IPA.