I too was unable to find the data in the 3rd Edition of Lehninger's Biochemistry.
A good source of such information usually is
Data for Biochemical Research, by Rex M. C Dawson, Daphne C. Elliott, William H. Elliott and Kenneth M. Jones (3rd Edition, 1986, Clarendon Press Oxford).
The first thing to note is that the ratios may be significantly
pH-dependent, and the source quoted by the OP does not specify what pH the data refer to.
The data in Column 2 below been taken from the above source, from which the data in Column 3 were calculated.
Adenine 0.13 ~ 7.7
Cytosine 0.58 ~ 1.7
Guanine 1.04 ~ 0.96
Uracil 0.17 ~ 5.9
Thymine 0.53 ~ 1.9
These data seem to have been substantially taken from an article by K. Burton in the second edition of DFBR.
K. Burton (1969) Spectral Data and pK values for purines, pyrimidines, nucleosides, and nucleotides in Data for Biochemical Research by Rex M. C Dawson, Daphne C. Elliott, William H. Elliott and Kenneth M. Jones (2rd Edition, Clarendon Press Oxford).