The value P(E | H) is the probability that a person not connected with the crime would have the same matching blood type as that found at the scene. This is 1/1000 because we are assuming that blood type occurs in 1 in a 1000 people. In 10,000 people we expect about 10 to have the matching blood type. In the absence of any other evidence any one of the 10 people is no more likely than the other 9 to have committed the crime. This is where the approximate 9/10 probability of innocence comes in once you have the evidence. i.e. this is where P(H | E) is about 0.9 (the fact it is actually 0.91 is due to some more explicit assumptions needed as shown in the calculatons on page 124).
If 10 out of 10,000 have the same blood group, can we say that the 10 comprises of:
1) 9 innocents
2) 1 culprit
Hence if we have 9 innocents, and we want to get the probability of seeing the evidence if the person is innocent, so instead of taking P(E|H) = 10/10,000 = 1/1000 = (0.001)
can we take P(E|H) = 9/10,000 (0.0009)? to be more accurate.