Mono recordings - two questions...

for solo vocal with instrument mono is better otherwise not.

The reason you could find a Julie London LP in mono is because most of her work was done in the mono era of LP production (early 50s to mid-60s).  I very much doubt that you can find mono recordings of the artists you list in your last sentence, because they are very much with us in the here and now, in stereo.  I have some Julie London mono LPs; I agree it works well in mono.  But I would not agree that you need to limit yourself to solo performances; most small group jazz and chamber music does very well in mono, and somehow the brain picks up spatial cues from mono surprisingly well.

I thought at one point in reading your post that you were going to ask about listening to mono through one speaker of a stereo pair (assuming your system is basically stereo), vs listening to a stereo pair of speakers each of which is reproducing the same mono signal, simultaneously.  Anyway, I take the latter approach.  Then there is the endless debate about using a mono cartridge vs a mono switch on your preamplifier, or both.  It is true that mono signals are generated from lateral movement of the stylus, exclusively. Vertical motion generates noise from the LP groove, only. Whereas true stereo reproduction requires that the music signal is encoded in both planes.  Thus stereo cartridges tend to reproduce noise from mono LPs, and many think that using a mono cartridge or a mono switch is the only way to go. (Either way, you are cancelling the signal due to vertical motion of the stylus by bridging.)

Glad to hear you are getting interested in Mono and the Julie London disc is a good place to start. However one of the great things about Mono is that you do not have to be restricted to re-issues and instead you can go and dig in a wealth of dirt cheap 50s and early 60s material

There is a ton of depth and imaging in a good Mono recording although all in one dimension of course. Reproduced well you can place all of the instruments in relative position to one another

One of the other benefits of mono is actually wider frequency range (i.e. more and cleaner high frequencies) than stereo. The reason for this is that it is easier to design a mono cutting head, it weighs less and can cut the high frequencies more accurately than the corresponding stereo. This is one of the reasons why mono versions of discs that exist in both mono and stereo may be better sounding (and of course the mixes are different)

Anyway on to recommendations.

• One of my favorite "guilty pleasures" is anything by the Chordettes -- superb small ensemble harmony and cute arrangements -- one of Ken Kessler’s go to discs https://www.youtube.com/watch?v=IaHS1K3H66c
  
• While I only have her work on CD I’d look for anything by Fern Jones https://www.youtube.com/watch?v=hjCVZNm4Prs (seems she only released one LP, but the recent compilation is all in mono and is cheap, 
• You should get into classical -- this is where the real mono magic (and the mega big bucks recordings) is -- maybe start with the original Dieskau/Moore Winterreise https://www.youtube.com/watch?v=KKkqqL0O9lc
• And then move on to Johanna Martzy (watch out for your check book, don’t ask what this repressing costs) https://www.youtube.com/watch?v=KKkqqL0O9lc

Anyway good luck!