Reddit mentions of Handbook of the International Phonetic Association: A Guide to the Use of the International Phonetic Alphabet

You'd probably be better off giving them the Handbook for the IPA. IPA flashcards would be cool, but the IPA is pretty straightforward, and I would fear that the IPA flashcards would become obsolete a week in. The Handbook basically includes phonologies for every language and is wonderfully useful in the future for ESL classes since you can look up the phonology of an ESL student's native tongue and get some more perspective.

I don't think it's necessary, no. But if you're the kind of person that benefits from this kind of thing, it can be beneficial. Sorry for being tautological =)

If you was about to learn new swimming styles, are you the kind of person who would go to the library and research on books about sports science as applied to swimming? If learning the guitar, do you brush up on music theory and try to understand the patterns of notes and chords in the fretboard? Before reading poetry, do you study about metrical forms and the history of styles? If you're that kind of person, you might have fun studying some linguistics and then trying to apply it to the languages you learn. Ocasionally it might even be useful!

As for book recommendations, I think one of the most useful areas you can investigate is phonetics and phonology, the study of linguistic sounds. Spoken languages are made of sounds that you hear, and you decode writing into sort-of "mental sounds" (assuming you're not congenitally deaf). Unfortunately these two processes have complications.

• Adult non-natives often fail to perceive and produce sounds in the new language (L2) that aren't present in their mother tongue (L1).
  
  Due to personal experience, I'm a supporter of the theory that this can be remedied by explicitly learning to notice the different sounds (Schmidt's Noticing Hypothesis). That means you need to understand how you yourself produce linguistic sounds, so that you can adapt your vocal gestures to those of other languages.
  
  To be able to do that, first of all, you have to learn the International Phonetic Alphabet (IPA). No, seriously, you need it. The Wikipedia articles are quite decent, and so is IPA's own handbook. Wikipedia also has recordings for all sounds—and so does this interactive table. Some beginners think of IPA as a writing system, and try to learn to "read" it as a whole. This is a mistake. Think of it as a table of possible sounds, classified on various dimensions (in the case of consonants, which are easier to introspect, there are three: place, manner, and voice). Once you understand how each dimension explains part of a gesture, the values of each symbol become a recipe of how to produce it: do this with my tongue here, turn on voice, and… voilà!
  
  If you're familiar with language X, look at the Wikipedia article "Phonology of X", and try to refer to the table and understand it by reproducing the sounds. (In the case of English, be sure to locate your own dialect/accent). Then try to understand the corresponding articles for the languages you're learning. You don't need to care about the rest of the table.
  
  As you get familiar with the IPA, try to learn the basics of articulatory phonetics and phonology. Online articles are probably good enough to help with language learning, but if you like technical books and want to dig deeper, I benefited a lot this and this and this one. If and only if you like physics, then this too.
  
  
• It's nontrivial to deduce the sounds from the writing (witness how much trouble computer people have with speech synthesis). All writing systems assume that you already know the language. Some assume just a little, like Finnish or Czech (and are therefore foreign-friendly); and some are basically unpredictable, like English and French; most are somewhere in-between.
  
  The best solution to this is to make sure you get lots and lots and lots of exposure to the spoken language. If you can learn the spoken language before writing, so much the better (it's how natives learn, after all). If you're curious about how writing systems work, I'd recommend Rogers as a first stop, but I think the Cree/Blackfoot sillabaries shouldn't give you any trouble.