I've got a certain weakness for eccentric phrasebooks, being someone who both loves languages and is a lunatic. And it was at Barter Books that I discovered the best eccentric phrasebook I've ever come across: Handbook of Gaelic Phrases and Sentences by Alexander MacFarlane, M.A.
I think it was first published in 1949, and the Gaelic being referred to is Scottish (rather than Irish, which should properly be called Irish, but some people insist on trolling me by getting this wrong). The humour lies largely in the fact that it equips you for very, very specific situations.

"Are you going? 'Tis not before time"
"Let us go and fish for lythe. We shall troll along the coast"
A selection of even more of my favourite phrases:
Lilt a tune for us, and we shall dance a reel (Thoir a steach gual agus dannsaidh sinn righle)

Are you feeling spry to-day? (A bheil surd math oirbh an diurgh?)

-Hand me that hat-box (Sin thugam am bosdan sin)
-Why do you want it? (Car son a tha sibh 'g a iarraidh?)
-To lay away my cap (Ach an gleidh mi seachad mo churrachd)

-Hand me over those moccasins (Sin a nall na cuaranan sin)
-They will not help you much. They are like a singed cat. (Cha mhor is fheairrde thu iad. Tha iad coltach ri cat air a dhathadh)

-Is John recovering? (A bheil Iain a' dol am feabhus?)
-He is still spared (Tha e mu seach fhathast)

Beat him (Gabh air)

An empty bag will not stand upright (Cha sheas poca falamh)

And finally the entire exchange on this page, the Gaelic of which I shall not transcribe for fear it'd drive me mad:

-Phew! You have it too hot!
-It is better hot.
-Perhaps it is, but there is a difference between sense and madness.
-So they say.
-That is like you and me.
-Is it? How do you make that out?
-This way: I have the sense.
-Well! well! and the madness is on me?
-How neatly you guessed it!
-A learned man understands a hint, Mary.
-It was not your skill in reading, John, that made you so clever.
-You know: a dog know when it is guilty.
-We shall leave it like that just now, Mary.

I think this book is the best thing I've ever owned. It makes me want to start a library of mental phrasebooks. I also bought the Max and Moritz translated into Shetlandese from Plattdeutsch that I was on about the other week. I'm a happy little potato.