Get 'Waka/Jawaka' for more in the 'Hot Rats' instrumental vein: a fine album.
Of the Mothers period, I'd heartily recommend 'Absolutely Free' and 'Lumpy Gravy' in addition to the obvious 'Freak Out!' and 'Money'. The live album at Fillmore East is excellent too.
'Apostrophe' is my clear fave of the solo albums. And 'Guitar' is the obvious choice for hearing the man's considerable axe skills bereft of his very individual humour.
The 'Cheep Thrills' samplers are great value and give a good cross-section of Zappa's work. Start there, my friend.