Hard..

Lotta books on these lists that I'd have in a top 25 or 50.. Gonna leave out manga because it's hard enough to pick 5 without considering Adachi Mitsuru.
Patrick O'Brian - Aubrey/Maturin series, Book 1 (actually the 2nd book in the series, Post Captain but like someone said, the 21 book series is like one very long book to me too so M&C is the beginning)
P.G. Wodehouse - Leave It To Psmith
Dorothy Dunnett - Pawn in Frankincense, Vol 5 of The Game of Kings
C.J. Cherryh - The Pride of Chanur
Sir Walter Scott - Ivanhoe