Donald Spoto has a reputation as a good biographer, and it is well deserved. Marilyn Monroe: The Biography is written intelligently and concisely and is positively saturated in interesting details.
Everybody wants a piece of poor Marilyn - the sheer volume of books written on this tragic woman attest to that. As another reviewer has stated, this biography may be closest to the truth with its straightforward approach based solidly in available facts. Note, however, I say "closest". There are some glaring errors (most notably Marilyn and Sidney Skolsky meeting with Jean Harlow's mother on July 15, 1962, although June 11, 1958 is the date printed on Mother Jean's death certificate) and an annoying lack of notes in an otherwise meticulous list of sources for events such as Marilyn and Joe DiMaggio's purported plans for remarriage in 1962. Do not let these mistakes deter you, however, as this is an otherwise well researched and trustworthy book, especially as far as dispelling long standing and oft repeated myths.
To conclude: read and enjoy this biography, but read a couple of others also. It is impossible to know exactly what transpired in many situations, simply because we weren't there, but it is possible to gather as may facts as possible and draw our own conclusions. After all, that's part of the fun of reading a biography - especially on someone as enigmatic and well documented as Marilyn.